This part of the reference documentation covers the Spring Framework’s integration with
a number of Java EE (and related) technologies.

1. Remoting and web services using Spring

1.1. Introduction

Spring features integration classes for remoting support using various technologies. The
remoting support eases the development of remote-enabled services, implemented by your
usual (Spring) POJOs. Currently, Spring supports the following remoting technologies:

Remote Method Invocation (RMI). Through the use of the RmiProxyFactoryBean and
the RmiServiceExporter Spring supports both traditional RMI (with java.rmi.Remote
interfaces and java.rmi.RemoteException) and transparent remoting via RMI invokers
(with any Java interface).

Spring’s HTTP invoker. Spring provides a special remoting strategy which allows
for Java serialization via HTTP, supporting any Java interface (just like the RMI
invoker). The corresponding support classes are HttpInvokerProxyFactoryBean and
HttpInvokerServiceExporter.

Hessian. By using Spring’s HessianProxyFactoryBean and the
HessianServiceExporter you can transparently expose your services using the
lightweight binary HTTP-based protocol provided by Caucho.

JAX-WS. Spring provides remoting support for web services via JAX-WS (the
successor of JAX-RPC, as introduced in Java EE 5 and Java 6).

JMS. Remoting using JMS as the underlying protocol is supported via the
JmsInvokerServiceExporter and JmsInvokerProxyFactoryBean classes.

AMQP. Remoting using AMQP as the underlying protocol is supported by the Spring
AMQP project.

While discussing the remoting capabilities of Spring, we’ll use the following domain
model and corresponding services:

We will start exposing the service to a remote client by using RMI and talk a bit about
the drawbacks of using RMI. We’ll then continue to show an example using Hessian as the
protocol.

1.2. Exposing services using RMI

Using Spring’s support for RMI, you can transparently expose your services through the
RMI infrastructure. After having this set up, you basically have a configuration similar
to remote EJBs, except for the fact that there is no standard support for security
context propagation or remote transaction propagation. Spring does provide hooks for
such additional invocation context when using the RMI invoker, so you can for example
plug in security frameworks or custom security credentials here.

1.2.1. Exporting the service using the RmiServiceExporter

Using the RmiServiceExporter, we can expose the interface of our AccountService object
as RMI object. The interface can be accessed by using RmiProxyFactoryBean, or via
plain RMI in case of a traditional RMI service. The RmiServiceExporter explicitly
supports the exposing of any non-RMI services via RMI invokers.

Of course, we first have to set up our service in the Spring container:

<beanclass="org.springframework.remoting.rmi.RmiServiceExporter"><!-- does not necessarily have to be the same name as the bean to be exported --><propertyname="serviceName"value="AccountService"/><propertyname="service"ref="accountService"/><propertyname="serviceInterface"value="example.AccountService"/><!-- defaults to 1099 --><propertyname="registryPort"value="1199"/></bean>

As you can see, we’re overriding the port for the RMI registry. Often, your application
server also maintains an RMI registry and it is wise to not interfere with that one.
Furthermore, the service name is used to bind the service under. So right now, the
service will be bound at 'rmi://HOST:1199/AccountService'. We’ll use the URL later on
to link in the service at the client side.

The servicePort property has been omitted (it defaults to 0). This means that an
anonymous port will be used to communicate with the service.

1.2.2. Linking in the service at the client

Our client is a simple object using the AccountService to manage accounts:

That’s all we need to do to support the remote account service on the client. Spring
will transparently create an invoker and remotely enable the account service through the
RmiServiceExporter. At the client we’re linking it in using the RmiProxyFactoryBean.

1.3. Using Hessian to remotely call services via HTTP

Hessian offers a binary HTTP-based remoting protocol. It is developed by Caucho and more
information about Hessian itself can be found at http://www.caucho.com.

1.3.1. Wiring up the DispatcherServlet for Hessian and co.

Hessian communicates via HTTP and does so using a custom servlet. Using Spring’s
DispatcherServlet principles, as known from Spring Web MVC usage, you can easily wire
up such a servlet exposing your services. First we’ll have to create a new servlet in
your application (this is an excerpt from 'web.xml'):

You’re probably familiar with Spring’s DispatcherServlet principles and if so, you
know that now you’ll have to create a Spring container configuration resource named
'remoting-servlet.xml' (after the name of your servlet) in the 'WEB-INF' directory.
The application context will be used in the next section.

Alternatively, consider the use of Spring’s simpler HttpRequestHandlerServlet. This
allows you to embed the remote exporter definitions in your root application context (by
default in 'WEB-INF/applicationContext.xml'), with individual servlet definitions
pointing to specific exporter beans. Each servlet name needs to match the bean name of
its target exporter in this case.

1.3.2. Exposing your beans by using the HessianServiceExporter

In the newly created application context called remoting-servlet.xml, we’ll create a
HessianServiceExporter exporting your services:

Now we’re ready to link in the service at the client. No explicit handler mapping is
specified, mapping request URLs onto services, so BeanNameUrlHandlerMapping will be
used: Hence, the service will be exported at the URL indicated through its bean name
within the containing DispatcherServlet's mapping (as defined above):
'http://HOST:8080/remoting/AccountService'.

In the latter case, define a corresponding servlet for this exporter in 'web.xml',
with the same end result: The exporter getting mapped to the request path
/remoting/AccountService. Note that the servlet name needs to match the bean name of
the target exporter.

1.3.3. Linking in the service on the client

Using the HessianProxyFactoryBean we can link in the service at the client. The same
principles apply as with the RMI example. We’ll create a separate bean factory or
application context and mention the following beans where the SimpleObject is using
the AccountService to manage accounts:

1.3.4. Applying HTTP basic authentication to a service exposed through Hessian

One of the advantages of Hessian is that we can easily apply HTTP basic authentication,
because both protocols are HTTP-based. Your normal HTTP server security mechanism can
easily be applied through using the web.xml security features, for example. Usually,
you don’t use per-user security credentials here, but rather shared credentials defined
at the HessianProxyFactoryBean level (similar to a JDBC DataSource).

This is an example where we explicitly mention the BeanNameUrlHandlerMapping and set
an interceptor allowing only administrators and operators to call the beans mentioned in
this application context.

Of course, this example doesn’t show a flexible kind of security infrastructure. For
more options as far as security is concerned, have a look at the Spring Security project
at http://projects.spring.io/spring-security/.

1.4. Exposing services using HTTP invokers

As opposed to Hessian, which are both lightweight protocols using their own slim
serialization mechanisms, Spring HTTP invokers use the standard Java serialization
mechanism to expose services through HTTP. This has a huge advantage if your arguments
and return types are complex types that cannot be serialized using the serialization
mechanisms Hessian uses (refer to the next section for more considerations when
choosing a remoting technology).

Under the hood, Spring uses either the standard facilities provided by the JDK or
Apache HttpComponents to perform HTTP calls. Use the latter if you need more
advanced and easier-to-use functionality. Refer to
hc.apache.org/httpcomponents-client-ga/
for more information.

Be aware of vulnerabilities due to unsafe Java deserialization:
Manipulated input streams could lead to unwanted code execution on the server
during the deserialization step. As a consequence, do not expose HTTP invoker
endpoints to untrusted clients but rather just between your own services.
In general, we strongly recommend any other message format (e.g. JSON) instead.

1.4.1. Exposing the service object

Setting up the HTTP invoker infrastructure for a service object resembles closely the
way you would do the same using Hessian. Just as Hessian support provides the
HessianServiceExporter, Spring’s HttpInvoker support provides the
org.springframework.remoting.httpinvoker.HttpInvokerServiceExporter.

To expose the AccountService (mentioned above) within a Spring Web MVC
DispatcherServlet, the following configuration needs to be in place in the
dispatcher’s application context:

If you are running outside of a servlet container and are using Oracle’s Java 6, then you
can use the built-in HTTP server implementation. You can configure the
SimpleHttpServerFactoryBean together with a SimpleHttpInvokerServiceExporter as is
shown in this example:

1.4.2. Linking in the service at the client

Again, linking in the service from the client much resembles the way you would do it
when using Hessian. Using a proxy, Spring will be able to translate your calls to
HTTP POST requests to the URL pointing to the exported service.

As mentioned before, you can choose what HTTP client you want to use. By default, the
HttpInvokerProxy uses the JDK’s HTTP functionality, but you can also use the Apache
HttpComponents client by setting the httpInvokerRequestExecutor property:

1.5. Web services

Spring provides full support for standard Java web services APIs:

Exposing web services using JAX-WS

Accessing web services using JAX-WS

In addition to stock support for JAX-WS in Spring Core, the Spring portfolio also
features Spring Web Services, a solution for
contract-first, document-driven web services - highly recommended for building modern,
future-proof web services.

1.5.1. Exposing servlet-based web services using JAX-WS

Spring provides a convenient base class for JAX-WS servlet endpoint implementations -
SpringBeanAutowiringSupport. To expose our AccountService we extend Spring’s
SpringBeanAutowiringSupport class and implement our business logic here, usually
delegating the call to the business layer. We’ll simply use Spring’s @Autowired
annotation for expressing such dependencies on Spring-managed beans.

/**
* JAX-WS compliant AccountService implementation that simply delegates
* to the AccountService implementation in the root web application context.
*
* This wrapper class is necessary because JAX-WS requires working with dedicated
* endpoint classes. If an existing service needs to be exported, a wrapper that
* extends SpringBeanAutowiringSupport for simple Spring bean autowiring (through
* the @Autowired annotation) is the simplest JAX-WS compliant way.
*
* This is the class registered with the server-side JAX-WS implementation.
* In the case of a Java EE 5 server, this would simply be defined as a servlet
* in web.xml, with the server detecting that this is a JAX-WS endpoint and reacting
* accordingly. The servlet name usually needs to match the specified WS service name.
*
* The web service engine manages the lifecycle of instances of this class.
* Spring bean references will just be wired in here.
*/importorg.springframework.web.context.support.SpringBeanAutowiringSupport;
@WebService(serviceName="AccountService")
publicclassAccountServiceEndpointextends SpringBeanAutowiringSupport {
@Autowiredprivate AccountService biz;
@WebMethodpublicvoid insertAccount(Account acc) {
biz.insertAccount(acc);
}
@WebMethodpublic Account[] getAccounts(String name) {
return biz.getAccounts(name);
}
}

Our AccountServiceEndpoint needs to run in the same web application as the Spring
context to allow for access to Spring’s facilities. This is the case by default in Java
EE 5 environments, using the standard contract for JAX-WS servlet endpoint deployment.
See Java EE 5 web service tutorials for details.

1.5.2. Exporting standalone web services using JAX-WS

The built-in JAX-WS provider that comes with Oracle’s JDK supports exposure of web
services using the built-in HTTP server that’s included in the JDK as well. Spring’s
SimpleJaxWsServiceExporter detects all @WebService annotated beans in the Spring
application context, exporting them through the default JAX-WS server (the JDK HTTP
server).

In this scenario, the endpoint instances are defined and managed as Spring beans
themselves; they will be registered with the JAX-WS engine but their lifecycle will be
up to the Spring application context. This means that Spring functionality like explicit
dependency injection may be applied to the endpoint instances. Of course,
annotation-driven injection through @Autowired will work as well.

The AccountServiceEndpoint may derive from Spring’s SpringBeanAutowiringSupport but
doesn’t have to since the endpoint is a fully Spring-managed bean here. This means that
the endpoint implementation may look like as follows, without any superclass declared -
and Spring’s @Autowired configuration annotation still being honored:

1.5.3. Exporting web services using the JAX-WS RI’s Spring support

Oracle’s JAX-WS RI, developed as part of the GlassFish project, ships Spring support as
part of its JAX-WS Commons project. This allows for defining JAX-WS endpoints as
Spring-managed beans, similar to the standalone mode discussed in the previous section -
but this time in a Servlet environment. Note that this is not portable in a Java EE 5
environment; it is mainly intended for non-EE environments such as Tomcat, embedding the
JAX-WS RI as part of the web application.

The difference to the standard style of exporting servlet-based endpoints is that the
lifecycle of the endpoint instances themselves will be managed by Spring here, and that
there will be only one JAX-WS servlet defined in web.xml. With the standard Java EE 5
style (as illustrated above), you’ll have one servlet definition per service endpoint,
with each endpoint typically delegating to Spring beans (through the use of
@Autowired, as shown above).

1.5.4. Accessing web services using JAX-WS

Spring provides two factory beans to create JAX-WS web service proxies, namely
LocalJaxWsServiceFactoryBean and JaxWsPortProxyFactoryBean. The former can only
return a JAX-WS service class for us to work with. The latter is the full-fledged
version that can return a proxy that implements our business service interface. In this
example we use the latter to create a proxy for the AccountService endpoint (again):

Where serviceInterface is our business interface the clients will use.
wsdlDocumentUrl is the URL for the WSDL file. Spring needs this a startup time to
create the JAX-WS Service. namespaceUri corresponds to the targetNamespace in the
.wsdl file. serviceName corresponds to the service name in the .wsdl file. portName
corresponds to the port name in the .wsdl file.

Accessing the web service is now very easy as we have a bean factory for it that will
expose it as AccountService interface. We can wire this up in Spring:

The above is slightly simplified in that JAX-WS requires endpoint interfaces
and implementation classes to be annotated with @WebService, @SOAPBinding etc
annotations. This means that you cannot (easily) use plain Java interfaces and
implementation classes as JAX-WS endpoint artifacts; you need to annotate them
accordingly first. Check the JAX-WS documentation for details on those requirements.

1.6. JMS

It is also possible to expose services transparently using JMS as the underlying
communication protocol. The JMS remoting support in the Spring Framework is pretty basic
- it sends and receives on the same thread and in the same non-transactionalSession, and as such throughput will be very implementation dependent. Note that these
single-threaded and non-transactional constraints apply only to Spring’s JMS
remoting support. See JMS (Java Message Service) for information on Spring’s rich support for JMS-based
messaging.

The following interface is used on both the server and the client side.

1.6.2. Client-side configuration

The client merely needs to create a client-side proxy that will implement the agreed
upon interface ( CheckingAccountService). The resulting object created off the back of
the following bean definition can be injected into other client side objects, and the
proxy will take care of forwarding the call to the server-side object via JMS.

1.7. AMQP

1.8. Auto-detection is not implemented for remote interfaces

The main reason why auto-detection of implemented interfaces does not occur for remote
interfaces is to avoid opening too many doors to remote callers. The target object might
implement internal callback interfaces like InitializingBean or DisposableBean which
one would not want to expose to callers.

Offering a proxy with all interfaces implemented by the target usually does not matter
in the local case. But when exporting a remote service, you should expose a specific
service interface, with specific operations intended for remote usage. Besides internal
callback interfaces, the target might implement multiple business interfaces, with just
one of them intended for remote exposure. For these reasons, we require such a
service interface to be specified.

This is a trade-off between configuration convenience and the risk of accidental
exposure of internal methods. Always specifying a service interface is not too much
effort, and puts you on the safe side regarding controlled exposure of specific methods.

1.9. Considerations when choosing a technology

Each and every technology presented here has its drawbacks. You should carefully
consider your needs, the services you are exposing and the objects you’ll be sending
over the wire when choosing a technology.

When using RMI, it’s not possible to access the objects through the HTTP protocol,
unless you’re tunneling the RMI traffic. RMI is a fairly heavy-weight protocol in that
it supports full-object serialization which is important when using a complex data model
that needs serialization over the wire. However, RMI-JRMP is tied to Java clients: It is
a Java-to-Java remoting solution.

Spring’s HTTP invoker is a good choice if you need HTTP-based remoting but also rely on
Java serialization. It shares the basic infrastructure with RMI invokers, just using
HTTP as transport. Note that HTTP invokers are not only limited to Java-to-Java remoting
but also to Spring on both the client and server side. (The latter also applies to
Spring’s RMI invoker for non-RMI interfaces.)

Hessian might provide significant value when operating in a heterogeneous environment,
because they explicitly allow for non-Java clients. However, non-Java support is still
limited. Known issues include the serialization of Hibernate objects in combination with
lazily-initialized collections. If you have such a data model, consider using RMI or
HTTP invokers instead of Hessian.

JMS can be useful for providing clusters of services and allowing the JMS broker to take
care of load balancing, discovery and auto-failover. By default: Java serialization is
used when using JMS remoting but the JMS provider could use a different mechanism for
the wire formatting, such as XStream to allow servers to be implemented in other
technologies.

Last but not least, EJB has an advantage over RMI in that it supports standard
role-based authentication and authorization and remote transaction propagation. It is
possible to get RMI invokers or HTTP invokers to support security context propagation as
well, although this is not provided by core Spring: There are just appropriate hooks for
plugging in third-party or custom solutions here.

1.10. REST Endpoints

The Spring Framework provides two choices for making calls to REST endpoints:

RestTemplate — the original Spring REST client with an API similar to other
template classes in Spring such as JdbcTemplate, JmsTemplate and others.
RestTemplate has a synchronous API and relies on blocking I/O which is okay for
client scenarios with low concurrency.

WebClient — reactive client with a functional,
fluent API from the spring-webflux module. It relies on non-blocking I/O which allows it
to support high concurrency more efficiently (i.e. using a small number of threads) than the
RestTemplate. WebClient is a natural fit for streaming scenarios.

1.10.1. RestTemplate

The RestTemplate provides a higher level API over HTTP client libraries. It makes it
easy to invoke REST endpoints in a single line. It exposes the following groups of
overloaded methods:

Table 1. RestTemplate methods

Method group

Description

getForObject

Retrieve a representation via GET.

getForEntity

Retrieve a ResponseEntity, i.e. status, headers, and body, via GET.

headForHeaders

Retrieve all headers for a resource via HEAD.

postForLocation

Create a new resource via POST and return the Location header from the response.

postForObject

Create a new resource via POST and return the representation from the response.

postForEntity

Create a new resource via POST and return the representation from the response.

put

Create or update a resource via PUT.

patchForObject

Update a resource via PATCH and return the representation from the response.
Note that the JDK HttpURLConnection does not support the PATCH but Apache
HttpComponents, and others do.

delete

Delete the resources at the specified URI via DELETE.

optionsForAllow

Retrieve allowed HTTP methods for a resource via ALLOW.

exchange

More generalized, and less opinionated version, of the above methods that provides extra
flexibility when needed. It accepts RequestEntity, including HTTP method, URL, headers,
and body as input, and returns a ResponseEntity.

These methods allow the use of ParameterizedTypeReference instead of Class to specify
a response type with generics.

execute

The most generalized way to perform a request, with full control over request
preparation and response extraction via callback interfaces.

Initialization

The default constructor uses java.net.HttpURLConnection to perform requests. You can
switch to a different HTTP library with an implementation of ClientHttpRequestFactory.
There is built-in support for the following:

Note that the java.net implementation for HTTP requests may raise an exception when
accessing the status of a response that represents an error (e.g. 401). If this is an
issue, switch to another HTTP client library.

URIs

Many of the RestTemplate methods accepts a URI template and URI template variables,
either as a String vararg, or as Map<String,String>.

The "Content-Type" header of the request does not need to be set explicitly. In most cases
a compatible message converter can be found based on the source Object type, and the chosen
message converter will set the content type accordingly. If necessary, you can use the
exchange methods to provide the "Content-Type" request header explicitly, and that in
turn will influence what message converter is selected.

On a GET, the body of the response is deserialized to an output Object:

The "Accept" header of the request does not need to be set explicitly. In most cases
a compatible message converter can be found based on the expected response type, which
then helps to populate the "Accept" header. If necessary, you can use the exchange
methods to provide the "Accept" header explicitly.

By default RestTemplate registers all built-in
message converters, depending on classpath checks that help
to determine what optional conversion libraries are present. You can also set the message
converters to use explicitly.

Message Conversion

The spring-web module contains the HttpMessageConverter contract for reading and
writing the body of HTTP requests and responses via InputStream and OutputStream.
HttpMessageConverter's are used on the client side, e.g. in the RestTemplate, and
also on the server side, e.g. in Spring MVC REST controllers.

Concrete implementations for the main media (MIME) types are provided in the framework
and are registered by default with the RestTemplate on the client-side and with
RequestMethodHandlerAdapter on the server-side (see
Configuring Message Converters).

The implementations of HttpMessageConverters are described in the following sections.
For all converters a default media type is used but can be overridden by setting the
supportedMediaTypes bean property

Table 2. HttpMessageConverter Implementations

MessageConverter

Description

StringHttpMessageConverter

An HttpMessageConverter implementation that can read and write Strings from the HTTP
request and response. By default, this converter supports all text media types (
text/*), and writes with a Content-Type of text/plain.

FormHttpMessageConverter

An HttpMessageConverter implementation that can read and write form data from the HTTP
request and response. By default, this converter reads and writes the media type
application/x-www-form-urlencoded. Form data is read from and written into a
MultiValueMap<String, String>.

ByteArrayHttpMessageConverter

An HttpMessageConverter implementation that can read and write byte arrays from the
HTTP request and response. By default, this converter supports all media types ( */*),
and writes with a Content-Type of application/octet-stream. This can be overridden
by setting the supportedMediaTypes property, and overriding getContentType(byte[]).

MarshallingHttpMessageConverter

An HttpMessageConverter implementation that can read and write XML using Spring’s
Marshaller and Unmarshaller abstractions from the org.springframework.oxm package.
This converter requires a Marshaller and Unmarshaller before it can be used. These
can be injected via constructor or bean properties. By default this converter supports (
text/xml) and ( application/xml).

MappingJackson2HttpMessageConverter

An HttpMessageConverter implementation that can read and write JSON using Jackson’s
ObjectMapper. JSON mapping can be customized as needed through the use of Jackson’s
provided annotations. When further control is needed, a custom ObjectMapper can be
injected through the ObjectMapper property for cases where custom JSON
serializers/deserializers need to be provided for specific types. By default this
converter supports ( application/json).

MappingJackson2XmlHttpMessageConverter

An HttpMessageConverter implementation that can read and write XML using
Jackson XML extension’s
XmlMapper. XML mapping can be customized as needed through the use of JAXB
or Jackson’s provided annotations. When further control is needed, a custom XmlMapper
can be injected through the ObjectMapper property for cases where custom XML
serializers/deserializers need to be provided for specific types. By default this
converter supports ( application/xml).

SourceHttpMessageConverter

An HttpMessageConverter implementation that can read and write
javax.xml.transform.Source from the HTTP request and response. Only DOMSource,
SAXSource, and StreamSource are supported. By default, this converter supports (
text/xml) and ( application/xml).

BufferedImageHttpMessageConverter

An HttpMessageConverter implementation that can read and write
java.awt.image.BufferedImage from the HTTP request and response. This converter reads
and writes the media type supported by the Java I/O API.

Jackson JSON Views

It is possible to specify a Jackson JSON View
to serialize only a subset of the object properties. For example:

Multipart

To send multipart data, you need to provide a MultiValueMap<String, ?> whose values are
either Objects representing part content, or HttpEntity representing the content and
headers for a part. MultipartBodyBuilder provides a convenient API to prepare a
multipart request:

In most cases you do not have to specify the Content-Type for each part. The content
type is determined automatically based on the HttpMessageConverter chosen to serialize it,
or in the case of a Resource based on the file extension. If necessary you can
explicitly provide the MediaType to use for each part through one fo the overloaded
builder part methods.

If the MultiValueMap contains at least one non-String value, which could also be
represent regular form data (i.e. "application/x-www-form-urlencoded"), you don’t have to
set the Content-Type to "multipart/form-data". This is always the case when using
MultipartBodyBuilder which ensures an HttpEntity wrapper.

1.10.2. Async RestTemplate

The AsyncRestTemplate is deprecated. For all use cases where the AsyncRestTemplate
is considered for use, please use the WebClient instead.

2. Enterprise JavaBeans (EJB) integration

2.1. Introduction

As a lightweight container, Spring is often considered an EJB replacement. We do believe
that for many if not most applications and use cases, Spring as a container, combined
with its rich supporting functionality in the area of transactions, ORM and JDBC access,
is a better choice than implementing equivalent functionality via an EJB container and
EJBs.

However, it is important to note that using Spring does not prevent you from using EJBs.
In fact, Spring makes it much easier to access EJBs and implement EJBs and functionality
within them. Additionally, using Spring to access services provided by EJBs allows the
implementation of those services to later transparently be switched between local EJB,
remote EJB, or POJO (plain old Java object) variants, without the client code having to
be changed.

In this chapter, we look at how Spring can help you access and implement EJBs. Spring
provides particular value when accessing stateless session beans (SLSBs), so we’ll begin
by discussing this.

2.2. Accessing EJBs

2.2.1. Concepts

To invoke a method on a local or remote stateless session bean, client code must
normally perform a JNDI lookup to obtain the (local or remote) EJB Home object, then use
a 'create' method call on that object to obtain the actual (local or remote) EJB object.
One or more methods are then invoked on the EJB.

To avoid repeated low-level code, many EJB applications use the Service Locator and
Business Delegate patterns. These are better than spraying JNDI lookups throughout
client code, but their usual implementations have significant disadvantages. For example:

Typically code using EJBs depends on Service Locator or Business Delegate singletons,
making it hard to test.

In the case of the Service Locator pattern used without a Business Delegate,
application code still ends up having to invoke the create() method on an EJB home,
and deal with the resulting exceptions. Thus it remains tied to the EJB API and the
complexity of the EJB programming model.

Implementing the Business Delegate pattern typically results in significant code
duplication, where we have to write numerous methods that simply call the same method
on the EJB.

The Spring approach is to allow the creation and use of proxy objects, normally
configured inside a Spring container, which act as codeless business delegates. You do
not need to write another Service Locator, another JNDI lookup, or duplicate methods in
a hand-coded Business Delegate unless you are actually adding real value in such code.

2.2.2. Accessing local SLSBs

Assume that we have a web controller that needs to use a local EJB. We’ll follow best
practice and use the EJB Business Methods Interface pattern, so that the EJB’s local
interface extends a non EJB-specific business methods interface. Let’s call this
business methods interface MyComponent.

publicinterfaceMyComponent {
...
}

One of the main reasons to use the Business Methods Interface pattern is to ensure that
synchronization between method signatures in local interface and bean implementation
class is automatic. Another reason is that it later makes it much easier for us to
switch to a POJO (plain old Java object) implementation of the service if it makes sense
to do so. Of course we’ll also need to implement the local home interface and provide an
implementation class that implements SessionBean and the MyComponent business
methods interface. Now the only Java coding we’ll need to do to hook up our web tier
controller to the EJB implementation is to expose a setter method of type MyComponent
on the controller. This will save the reference as an instance variable in the
controller:

We can subsequently use this instance variable in any business method in the controller.
Now assuming we are obtaining our controller object out of a Spring container, we can
(in the same context) configure a LocalStatelessSessionProxyFactoryBean instance,
which will be the EJB proxy object. The configuration of the proxy, and setting of the
myComponent property of the controller is done with a configuration entry such as:

There’s a lot of work happening behind the scenes, courtesy of the Spring AOP framework,
although you aren’t forced to work with AOP concepts to enjoy the results. The
myComponent bean definition creates a proxy for the EJB, which implements the business
method interface. The EJB local home is cached on startup, so there’s only a single JNDI
lookup. Each time the EJB is invoked, the proxy invokes the classname method on the
local EJB and invokes the corresponding business method on the EJB.

The myController bean definition sets the myComponent property of the controller
class to the EJB proxy.

Alternatively (and preferably in case of many such proxy definitions), consider using
the <jee:local-slsb> configuration element in Spring’s "jee" namespace:

This EJB access mechanism delivers huge simplification of application code: the web tier
code (or other EJB client code) has no dependence on the use of EJB. If we want to
replace this EJB reference with a POJO or a mock object or other test stub, we could
simply change the myComponent bean definition without changing a line of Java code.
Additionally, we haven’t had to write a single line of JNDI lookup or other EJB plumbing
code as part of our application.

Benchmarks and experience in real applications indicate that the performance overhead of
this approach (which involves reflective invocation of the target EJB) is minimal, and
is typically undetectable in typical use. Remember that we don’t want to make
fine-grained calls to EJBs anyway, as there’s a cost associated with the EJB
infrastructure in the application server.

There is one caveat with regards to the JNDI lookup. In a bean container, this class is
normally best used as a singleton (there simply is no reason to make it a prototype).
However, if that bean container pre-instantiates singletons (as do the various XML
ApplicationContext variants) you may have a problem if the bean container is loaded
before the EJB container loads the target EJB. That is because the JNDI lookup will be
performed in the init() method of this class and then cached, but the EJB will not
have been bound at the target location yet. The solution is to not pre-instantiate this
factory object, but allow it to be created on first use. In the XML containers, this is
controlled via the lazy-init attribute.

Although this will not be of interest to the majority of Spring users, those doing
programmatic AOP work with EJBs may want to look at LocalSlsbInvokerInterceptor.

2.2.3. Accessing remote SLSBs

Accessing remote EJBs is essentially identical to accessing local EJBs, except that the
SimpleRemoteStatelessSessionProxyFactoryBean or <jee:remote-slsb> configuration
element is used. Of course, with or without Spring, remote invocation semantics apply; a
call to a method on an object in another VM in another computer does sometimes have to
be treated differently in terms of usage scenarios and failure handling.

Spring’s EJB client support adds one more advantage over the non-Spring approach.
Normally it is problematic for EJB client code to be easily switched back and forth
between calling EJBs locally or remotely. This is because the remote interface methods
must declare that they throw RemoteException, and client code must deal with this,
while the local interface methods don’t. Client code written for local EJBs which needs
to be moved to remote EJBs typically has to be modified to add handling for the remote
exceptions, and client code written for remote EJBs which needs to be moved to local
EJBs, can either stay the same but do a lot of unnecessary handling of remote
exceptions, or needs to be modified to remove that code. With the Spring remote EJB
proxy, you can instead not declare any thrown RemoteException in your Business Method
Interface and implementing EJB code, have a remote interface which is identical except
that it does throw RemoteException, and rely on the proxy to dynamically treat the two
interfaces as if they were the same. That is, client code does not have to deal with the
checked RemoteException class. Any actual RemoteException that is thrown during the
EJB invocation will be re-thrown as the non-checked RemoteAccessException class, which
is a subclass of RuntimeException. The target service can then be switched at will
between a local EJB or remote EJB (or even plain Java object) implementation, without
the client code knowing or caring. Of course, this is optional; there is nothing
stopping you from declaring RemoteExceptions in your business interface.

2.2.4. Accessing EJB 2.x SLSBs versus EJB 3 SLSBs

Accessing EJB 2.x Session Beans and EJB 3 Session Beans via Spring is largely
transparent. Spring’s EJB accessors, including the <jee:local-slsb> and
<jee:remote-slsb> facilities, transparently adapt to the actual component at runtime.
They handle a home interface if found (EJB 2.x style), or perform straight component
invocations if no home interface is available (EJB 3 style).

3. JMS (Java Message Service)

3.1. Introduction

Spring provides a JMS integration framework that simplifies the use of the JMS API much
like Spring’s integration does for the JDBC API.

JMS can be roughly divided into two areas of functionality, namely the production and
consumption of messages. The JmsTemplate class is used for message production and
synchronous message reception. For asynchronous reception similar to Java EE’s
message-driven bean style, Spring provides a number of message listener containers that
are used to create Message-Driven POJOs (MDPs). Spring also provides a declarative way
of creating message listeners.

The package org.springframework.jms.core provides the core functionality for using
JMS. It contains JMS template classes that simplify the use of the JMS by handling the
creation and release of resources, much like the JdbcTemplate does for JDBC. The
design principle common to Spring template classes is to provide helper methods to
perform common operations and for more sophisticated usage, delegate the essence of the
processing task to user implemented callback interfaces. The JMS template follows the
same design. The classes offer various convenience methods for the sending of messages,
consuming a message synchronously, and exposing the JMS session and message producer to
the user.

The package org.springframework.jms.support provides JMSException translation
functionality. The translation converts the checked JMSException hierarchy to a
mirrored hierarchy of unchecked exceptions. If there are any provider specific
subclasses of the checked javax.jms.JMSException, this exception is wrapped in the
unchecked UncategorizedJmsException.

The package org.springframework.jms.support.converter provides a MessageConverter
abstraction to convert between Java objects and JMS messages.

The package org.springframework.jms.support.destination provides various strategies
for managing JMS destinations, such as providing a service locator for destinations
stored in JNDI.

The package org.springframework.jms.annotation provides the necessary infrastructure
to support annotation-driven listener endpoints using @JmsListener.

The package org.springframework.jms.config provides the parser implementation for the
jms namespace as well the java config support to configure listener containers and
create listener endpoints.

Finally, the package org.springframework.jms.connection provides an implementation of
the ConnectionFactory suitable for use in standalone applications. It also contains an
implementation of Spring’s PlatformTransactionManager for JMS (the cunningly named
JmsTransactionManager). This allows for seamless integration of JMS as a transactional
resource into Spring’s transaction management mechanisms.

3.2. Using Spring JMS

3.2.1. JmsTemplate

The JmsTemplate class is the central class in the JMS core package. It simplifies the
use of JMS since it handles the creation and release of resources when sending or
synchronously receiving messages.

Code that uses the JmsTemplate only needs to implement callback interfaces giving them
a clearly defined high level contract. The MessageCreator callback interface creates a
message given a Session provided by the calling code in JmsTemplate. In order to
allow for more complex usage of the JMS API, the callback SessionCallback provides the
user with the JMS session and the callback ProducerCallback exposes a Session and
MessageProducer pair.

The JMS API exposes two types of send methods, one that takes delivery mode, priority,
and time-to-live as Quality of Service (QOS) parameters and one that takes no QOS
parameters which uses default values. Since there are many send methods in
JmsTemplate, the setting of the QOS parameters have been exposed as bean properties to
avoid duplication in the number of send methods. Similarly, the timeout value for
synchronous receive calls is set using the property setReceiveTimeout.

Some JMS providers allow the setting of default QOS values administratively through the
configuration of the ConnectionFactory. This has the effect that a call to
MessageProducer's send method send(Destination destination, Message message) will
use different QOS default values than those specified in the JMS specification. In order
to provide consistent management of QOS values, the JmsTemplate must therefore be
specifically enabled to use its own QOS values by setting the boolean property
isExplicitQosEnabled to true.

For convenience, JmsTemplate also exposes a basic request-reply operation that allows
to send a message and wait for a reply on a temporary queue that is created as part of
the operation.

Instances of the JmsTemplate class are thread-safe once configured. This is
important because it means that you can configure a single instance of a JmsTemplate
and then safely inject this shared reference into multiple collaborators. To be
clear, the JmsTemplate is stateful, in that it maintains a reference to a
ConnectionFactory, but this state is not conversational state.

As of Spring Framework 4.1, JmsMessagingTemplate is built on top of JmsTemplate
and provides an integration with the messaging abstraction, i.e.
org.springframework.messaging.Message. This allows you to create the message to
send in generic manner.

3.2.2. Connections

The JmsTemplate requires a reference to a ConnectionFactory. The ConnectionFactory
is part of the JMS specification and serves as the entry point for working with JMS. It
is used by the client application as a factory to create connections with the JMS
provider and encapsulates various configuration parameters, many of which are vendor
specific such as SSL configuration options.

When using JMS inside an EJB, the vendor provides implementations of the JMS interfaces
so that they can participate in declarative transaction management and perform pooling
of connections and sessions. In order to use this implementation, Java EE containers
typically require that you declare a JMS connection factory as a resource-ref inside
the EJB or servlet deployment descriptors. To ensure the use of these features with the
JmsTemplate inside an EJB, the client application should ensure that it references the
managed implementation of the ConnectionFactory.

Caching Messaging Resources

The standard API involves creating many intermediate objects. To send a message the
following 'API' walk is performed

ConnectionFactory->Connection->Session->MessageProducer->send

Between the ConnectionFactory and the Send operation there are three intermediate
objects that are created and destroyed. To optimise the resource usage and increase
performance two implementations of ConnectionFactory are provided.

SingleConnectionFactory

Spring provides an implementation of the ConnectionFactory interface,
SingleConnectionFactory, that will return the same Connection on all
createConnection() calls and ignore calls to close(). This is useful for testing and
standalone environments so that the same connection can be used for multiple
JmsTemplate calls that may span any number of transactions. SingleConnectionFactory
takes a reference to a standard ConnectionFactory that would typically come from JNDI.

CachingConnectionFactory

The CachingConnectionFactory extends the functionality of SingleConnectionFactory
and adds the caching of Sessions, MessageProducers, and MessageConsumers. The initial
cache size is set to 1, use the property sessionCacheSize to increase the number of
cached sessions. Note that the number of actual cached sessions will be more than that
number as sessions are cached based on their acknowledgment mode, so there can be up to
4 cached session instances when sessionCacheSize is set to one, one for each
acknowledgment mode. MessageProducers and MessageConsumers are cached within their
owning session and also take into account the unique properties of the producers and
consumers when caching. MessageProducers are cached based on their destination.
MessageConsumers are cached based on a key composed of the destination, selector,
noLocal delivery flag, and the durable subscription name (if creating durable consumers).

3.2.3. Destination Management

Destinations, like ConnectionFactories, are JMS administered objects that can be stored
and retrieved in JNDI. When configuring a Spring application context you can use the
JNDI factory class JndiObjectFactoryBean / <jee:jndi-lookup> to perform dependency
injection on your object’s references to JMS destinations. However, often this strategy
is cumbersome if there are a large number of destinations in the application or if there
are advanced destination management features unique to the JMS provider. Examples of
such advanced destination management would be the creation of dynamic destinations or
support for a hierarchical namespace of destinations. The JmsTemplate delegates the
resolution of a destination name to a JMS destination object to an implementation of the
interface DestinationResolver. DynamicDestinationResolver is the default
implementation used by JmsTemplate and accommodates resolving dynamic destinations. A
JndiDestinationResolver is also provided that acts as a service locator for
destinations contained in JNDI and optionally falls back to the behavior contained in
DynamicDestinationResolver.

Quite often the destinations used in a JMS application are only known at runtime and
therefore cannot be administratively created when the application is deployed. This is
often because there is shared application logic between interacting system components
that create destinations at runtime according to a well-known naming convention. Even
though the creation of dynamic destinations is not part of the JMS specification, most
vendors have provided this functionality. Dynamic destinations are created with a name
defined by the user which differentiates them from temporary destinations and are often
not registered in JNDI. The API used to create dynamic destinations varies from provider
to provider since the properties associated with the destination are vendor specific.
However, a simple implementation choice that is sometimes made by vendors is to
disregard the warnings in the JMS specification and to use the TopicSession method
createTopic(String topicName) or the QueueSession method createQueue(String
queueName) to create a new destination with default destination properties. Depending
on the vendor implementation, DynamicDestinationResolver may then also create a
physical destination instead of only resolving one.

The boolean property pubSubDomain is used to configure the JmsTemplate with
knowledge of what JMS domain is being used. By default the value of this property is
false, indicating that the point-to-point domain, Queues, will be used. This property
used by JmsTemplate determines the behavior of dynamic destination resolution via
implementations of the DestinationResolver interface.

You can also configure the JmsTemplate with a default destination via the property
defaultDestination. The default destination will be used with send and receive
operations that do not refer to a specific destination.

3.2.4. Message Listener Containers

One of the most common uses of JMS messages in the EJB world is to drive message-driven
beans (MDBs). Spring offers a solution to create message-driven POJOs (MDPs) in a way
that does not tie a user to an EJB container. (See Asynchronous reception: Message-Driven POJOs
for detailed coverage of Spring’s MDP support.) As from Spring Framework 4.1, endpoint
methods can be simply annotated using @JmsListener see Annotation-driven listener endpoints for more
details.

A message listener container is used to receive messages from a JMS message queue and
drive the MessageListener that is injected into it. The listener container is
responsible for all threading of message reception and dispatches into the listener for
processing. A message listener container is the intermediary between an MDP and a
messaging provider, and takes care of registering to receive messages, participating in
transactions, resource acquisition and release, exception conversion and suchlike. This
allows you as an application developer to write the (possibly complex) business logic
associated with receiving a message (and possibly responding to it), and delegates
boilerplate JMS infrastructure concerns to the framework.

There are two standard JMS message listener containers packaged with Spring, each with
its specialised feature set.

SimpleMessageListenerContainer

This message listener container is the simpler of the two standard flavors. It creates a
fixed number of JMS sessions and consumers at startup, registers the listener using the
standard JMS MessageConsumer.setMessageListener() method, and leaves it up the JMS
provider to perform listener callbacks. This variant does not allow for dynamic adaption
to runtime demands or for participation in externally managed transactions.
Compatibility-wise, it stays very close to the spirit of the standalone JMS
specification - but is generally not compatible with Java EE’s JMS restrictions.

While SimpleMessageListenerContainer does not allow for the participation in externally
managed transactions, it does support native JMS transactions: simply switch the
'sessionTransacted' flag to 'true' or, in the namespace, set the 'acknowledge' attribute
to 'transacted': Exceptions thrown from your listener will lead to a rollback then, with
the message getting redelivered. Alternatively, consider using 'CLIENT_ACKNOWLEDGE' mode
which provides redelivery in case of an exception as well but does not use transacted
Sessions and therefore does not include any other Session operations (such as sending
response messages) in the transaction protocol.

The default 'AUTO_ACKNOWLEDGE' mode does not provide proper reliability guarantees.
Messages may get lost when listener execution fails (since the provider will automatically
acknowledge each message after listener invocation, with no exceptions to be propagated to
the provider) or when the listener container shuts down (this may be configured through
the 'acceptMessagesWhileStopping' flag). Make sure to use transacted sessions in case of
reliability needs, e.g. for reliable queue handling and durable topic subscriptions.

DefaultMessageListenerContainer

This message listener container is the one used in most cases. In contrast to
SimpleMessageListenerContainer, this container variant allows for dynamic adaptation
to runtime demands and is able to participate in externally managed transactions. Each
received message is registered with an XA transaction when configured with a
JtaTransactionManager; so processing may take advantage of XA transaction semantics.
This listener container strikes a good balance between low requirements on the JMS
provider, advanced functionality such as the participation in externally managed
transactions, and compatibility with Java EE environments.

The cache level of the container can be customized. Note that when no caching is enabled,
a new connection and a new session is created for each message reception. Combining this
with a non durable subscription with high loads may lead to message lost. Make sure to
use a proper cache level in such case.

This container also has recoverable capabilities when the broker goes down. By default,
a simple BackOff implementation retries every 5 seconds. It is possible to specify
a custom BackOff implementation for more fine-grained recovery options, see
ExponentialBackOff for an example.

Like its sibling SimpleMessageListenerContainer, DefaultMessageListenerContainer
supports native JMS transactions and also allows for customizing the acknowledgment mode.
This is strongly recommended over externally managed transactions if feasible for your
scenario: that is, if you can live with occasional duplicate messages in case of the
JVM dying. Custom duplicate message detection steps in your business logic may cover
such situations, e.g. in the form of a business entity existence check or a protocol
table check. Any such arrangements will be significantly more efficient than the
alternative: wrapping your entire processing with an XA transaction (through configuring
your DefaultMessageListenerContainer with an JtaTransactionManager), covering the
reception of the JMS message as well as the execution of the business logic in your
message listener (including database operations etc).

The default 'AUTO_ACKNOWLEDGE' mode does not provide proper reliability guarantees.
Messages may get lost when listener execution fails (since the provider will automatically
acknowledge each message before listener invocation) or when the listener container shuts
down (this may be configured through the 'acceptMessagesWhileStopping' flag). Make sure
to use transacted sessions in case of reliability needs, e.g. for reliable queue handling
and durable topic subscriptions.

3.2.5. Transaction management

Spring provides a JmsTransactionManager that manages transactions for a single JMS
ConnectionFactory. This allows JMS applications to leverage the managed transaction
features of Spring as described in Transaction Management.
The JmsTransactionManager performs local resource transactions, binding a JMS
Connection/Session pair from the specified ConnectionFactory to the thread.
JmsTemplate automatically detects such transactional resources and operates
on them accordingly.

In a Java EE environment, the ConnectionFactory will pool Connections and Sessions, so
those resources are efficiently reused across transactions. In a standalone environment,
using Spring’s SingleConnectionFactory will result in a shared JMS Connection, with
each transaction having its own independent Session. Alternatively, consider the use
of a provider-specific pooling adapter such as ActiveMQ’s PooledConnectionFactory
class.

JmsTemplate can also be used with the JtaTransactionManager and an XA-capable JMS
ConnectionFactory for performing distributed transactions. Note that this requires the
use of a JTA transaction manager as well as a properly XA-configured ConnectionFactory!
(Check your Java EE server’s / JMS provider’s documentation.)

Reusing code across a managed and unmanaged transactional environment can be confusing
when using the JMS API to create a Session from a Connection. This is because the
JMS API has only one factory method to create a Session and it requires values for the
transaction and acknowledgment modes. In a managed environment, setting these values is
the responsibility of the environment’s transactional infrastructure, so these values
are ignored by the vendor’s wrapper to the JMS Connection. When using the JmsTemplate
in an unmanaged environment you can specify these values through the use of the
properties sessionTransacted and sessionAcknowledgeMode. When using a
PlatformTransactionManager with JmsTemplate, the template will always be given a
transactional JMS Session.

3.3. Sending a Message

The JmsTemplate contains many convenience methods to send a message. There are send
methods that specify the destination using a javax.jms.Destination object and those
that specify the destination using a string for use in a JNDI lookup. The send method
that takes no destination argument uses the default destination.

This example uses the MessageCreator callback to create a text message from the
supplied Session object. The JmsTemplate is constructed by passing a reference to a
ConnectionFactory. As an alternative, a zero argument constructor and
connectionFactory is provided and can be used for constructing the instance in
JavaBean style (using a BeanFactory or plain Java code). Alternatively, consider
deriving from Spring’s JmsGatewaySupport convenience base class, which provides
pre-built bean properties for JMS configuration.

The method send(String destinationName, MessageCreator creator) lets you send a
message using the string name of the destination. If these names are registered in JNDI,
you should set the destinationResolver property of the template to an instance of
JndiDestinationResolver.

If you created the JmsTemplate and specified a default destination, the
send(MessageCreator c) sends a message to that destination.

3.3.1. Using Message Converters

In order to facilitate the sending of domain model objects, the JmsTemplate has
various send methods that take a Java object as an argument for a message’s data
content. The overloaded methods convertAndSend() and receiveAndConvert() in
JmsTemplate delegate the conversion process to an instance of the MessageConverter
interface. This interface defines a simple contract to convert between Java objects and
JMS messages. The default implementation SimpleMessageConverter supports conversion
between String and TextMessage, byte[] and BytesMesssage, and java.util.Map
and MapMessage. By using the converter, you and your application code can focus on the
business object that is being sent or received via JMS and not be concerned with the
details of how it is represented as a JMS message.

The sandbox currently includes a MapMessageConverter which uses reflection to convert
between a JavaBean and a MapMessage. Other popular implementation choices you might
implement yourself are Converters that use an existing XML marshalling package, such as
JAXB, Castor or XStream, to create a TextMessage representing the object.

To accommodate the setting of a message’s properties, headers, and body that can not be
generically encapsulated inside a converter class, the MessagePostProcessor interface
gives you access to the message after it has been converted, but before it is sent. The
example below demonstrates how to modify a message header and a property after a
java.util.Map is converted to a message.

3.3.2. SessionCallback and ProducerCallback

While the send operations cover many common usage scenarios, there are cases when you
want to perform multiple operations on a JMS Session or MessageProducer. The
SessionCallback and ProducerCallback expose the JMS Session and Session /
MessageProducer pair respectively. The execute() methods on JmsTemplate execute
these callback methods.

3.4. Receiving a message

3.4.1. Synchronous reception

While JMS is typically associated with asynchronous processing, it is possible to
consume messages synchronously. The overloaded receive(..) methods provide this
functionality. During a synchronous receive, the calling thread blocks until a message
becomes available. This can be a dangerous operation since the calling thread can
potentially be blocked indefinitely. The property receiveTimeout specifies how long
the receiver should wait before giving up waiting for a message.

3.4.2. Asynchronous reception: Message-Driven POJOs

Spring also supports annotated-listener endpoints through the use of the @JmsListener
annotation and provides an open infrastructure to register endpoints programmatically. This
is by far the most convenient way to setup an asynchronous receiver, see
Enable listener endpoint annotations for more details.

In a fashion similar to a Message-Driven Bean (MDB) in the EJB world, the Message-Driven
POJO (MDP) acts as a receiver for JMS messages. The one restriction (but see also below
for the discussion of the MessageListenerAdapter class) on an MDP is that it must
implement the javax.jms.MessageListener interface. Please also be aware that in the
case where your POJO will be receiving messages on multiple threads, it is important to
ensure that your implementation is thread-safe.

Once you’ve implemented your MessageListener, it’s time to create a message listener
container.

Find below an example of how to define and configure one of the message listener
containers that ships with Spring (in this case the DefaultMessageListenerContainer).

<!-- this is the Message Driven POJO (MDP) --><beanid="messageListener"class="jmsexample.ExampleListener"/><!-- and this is the message listener container --><beanid="jmsContainer"class="org.springframework.jms.listener.DefaultMessageListenerContainer"><propertyname="connectionFactory"ref="connectionFactory"/><propertyname="destination"ref="destination"/><propertyname="messageListener"ref="messageListener"/></bean>

Please refer to the Spring javadocs of the various message listener containers for a full
description of the features supported by each implementation.

3.4.3. SessionAwareMessageListener interface

The SessionAwareMessageListener interface is a Spring-specific interface that provides
a similar contract to the JMS MessageListener interface, but also provides the message
handling method with access to the JMS Session from which the Message was received.

You can choose to have your MDPs implement this interface (in preference to the standard
JMS MessageListener interface) if you want your MDPs to be able to respond to any
received messages (using the Session supplied in the onMessage(Message, Session)
method). All of the message listener container implementations that ship with Spring
have support for MDPs that implement either the MessageListener or
SessionAwareMessageListener interface. Classes that implement the
SessionAwareMessageListener come with the caveat that they are then tied to Spring
through the interface. The choice of whether or not to use it is left entirely up to you
as an application developer or architect.

Please note that the 'onMessage(..)' method of the SessionAwareMessageListener
interface throws JMSException. In contrast to the standard JMS MessageListener
interface, when using the SessionAwareMessageListener interface, it is the
responsibility of the client code to handle any exceptions thrown.

3.4.4. MessageListenerAdapter

The MessageListenerAdapter class is the final component in Spring’s asynchronous
messaging support: in a nutshell, it allows you to expose almost any class as a MDP
(there are of course some constraints).

Consider the following interface definition. Notice that although the interface extends
neither the MessageListener nor SessionAwareMessageListener interfaces, it can still
be used as a MDP via the use of the MessageListenerAdapter class. Notice also how the
various message handling methods are strongly typed according to the contents of the
various Message types that they can receive and handle.

In particular, note how the above implementation of the MessageDelegate interface (the
above DefaultMessageDelegate class) has no JMS dependencies at all. It truly is a
POJO that we will make into an MDP via the following configuration.

<!-- this is the Message Driven POJO (MDP) --><beanid="messageListener"class="org.springframework.jms.listener.adapter.MessageListenerAdapter"><constructor-arg><beanclass="jmsexample.DefaultMessageDelegate"/></constructor-arg></bean><!-- and this is the message listener container... --><beanid="jmsContainer"class="org.springframework.jms.listener.DefaultMessageListenerContainer"><propertyname="connectionFactory"ref="connectionFactory"/><propertyname="destination"ref="destination"/><propertyname="messageListener"ref="messageListener"/></bean>

Below is an example of another MDP that can only handle the receiving of JMS
TextMessage messages. Notice how the message handling method is actually called
'receive' (the name of the message handling method in a MessageListenerAdapter
defaults to 'handleMessage'), but it is configurable (as you will see below). Notice
also how the 'receive(..)' method is strongly typed to receive and respond only to JMS
TextMessage messages.

Please note that if the above 'messageListener' receives a JMS Message of a type
other than TextMessage, an IllegalStateException will be thrown (and subsequently
swallowed). Another of the capabilities of the MessageListenerAdapter class is the
ability to automatically send back a response Message if a handler method returns a
non-void value. Consider the interface and class:

If the above DefaultResponsiveTextMessageDelegate is used in conjunction with a
MessageListenerAdapter then any non-null value that is returned from the execution of
the 'receive(..)' method will (in the default configuration) be converted into a
TextMessage. The resulting TextMessage will then be sent to the Destination (if
one exists) defined in the JMS Reply-To property of the original Message, or the
default Destination set on the MessageListenerAdapter (if one has been configured);
if no Destination is found then an InvalidDestinationException will be thrown (and
please note that this exception will not be swallowed and will propagate up the
call stack).

3.4.5. Processing messages within transactions

Invoking a message listener within a transaction only requires reconfiguration of the
listener container.

Local resource transactions can simply be activated through the sessionTransacted flag
on the listener container definition. Each message listener invocation will then operate
within an active JMS transaction, with message reception rolled back in case of listener
execution failure. Sending a response message (via SessionAwareMessageListener) will
be part of the same local transaction, but any other resource operations (such as
database access) will operate independently. This usually requires duplicate message
detection in the listener implementation, covering the case where database processing
has committed but message processing failed to commit.

For participating in an externally managed transaction, you will need to configure a
transaction manager and use a listener container which supports externally managed
transactions: typically DefaultMessageListenerContainer.

To configure a message listener container for XA transaction participation, you’ll want
to configure a JtaTransactionManager (which, by default, delegates to the Java EE
server’s transaction subsystem). Note that the underlying JMS ConnectionFactory needs to
be XA-capable and properly registered with your JTA transaction coordinator! (Check your
Java EE server’s configuration of JNDI resources.) This allows message reception as well
as e.g. database access to be part of the same transaction (with unified commit
semantics, at the expense of XA transaction log overhead).

3.5. Support for JCA Message Endpoints

Beginning with version 2.5, Spring also provides support for a JCA-based
MessageListener container. The JmsMessageEndpointManager will attempt to
automatically determine the ActivationSpec class name from the provider’s
ResourceAdapter class name. Therefore, it is typically possible to just provide
Spring’s generic JmsActivationSpecConfig as shown in the following example.

The specified WorkManager may also point to an environment-specific thread pool -
typically through SimpleTaskWorkManager's "asyncTaskExecutor" property. Consider
defining a shared thread pool for all your ResourceAdapter instances if you happen to
use multiple adapters.

In some environments (e.g. WebLogic 9 or above), the entire ResourceAdapter object may
be obtained from JNDI instead (using <jee:jndi-lookup>). The Spring-based message
listeners can then interact with the server-hosted ResourceAdapter, also using the
server’s built-in WorkManager.

Please consult the javadoc for JmsMessageEndpointManager, JmsActivationSpecConfig,
and ResourceAdapterFactoryBean for more details.

Spring also provides a generic JCA message endpoint manager which is not tied to JMS:
org.springframework.jca.endpoint.GenericMessageEndpointManager. This component allows
for using any message listener type (e.g. a CCI MessageListener) and any
provider-specific ActivationSpec object. Check out your JCA provider’s documentation to
find out about the actual capabilities of your connector, and consult
GenericMessageEndpointManager's javadoc for the Spring-specific configuration details.

JCA-based message endpoint management is very analogous to EJB 2.1 Message-Driven Beans;
it uses the same underlying resource provider contract. Like with EJB 2.1 MDBs, any
message listener interface supported by your JCA provider can be used in the Spring
context as well. Spring nevertheless provides explicit 'convenience' support for JMS,
simply because JMS is the most common endpoint API used with the JCA endpoint management
contract.

3.6. Annotation-driven listener endpoints

The easiest way to receive a message asynchronously is to use the annotated listener
endpoint infrastructure. In a nutshell, it allows you to expose a method of a managed
bean as a JMS listener endpoint.

The idea of the example above is that whenever a message is available on the
javax.jms.Destination "myDestination", the processOrder method is invoked
accordingly (in this case, with the content of the JMS message similarly to
what the MessageListenerAdapter
provides).

The annotated endpoint infrastructure creates a message listener container
behind the scenes for each annotated method, using a JmsListenerContainerFactory.
Such a container is not registered against the application context but can be easily
located for management purposes using the JmsListenerEndpointRegistry bean.

@JmsListener is a repeatable annotation on Java 8, so it is possible to associate
several JMS destinations to the same method by adding additional @JmsListener
declarations to it.

3.6.1. Enable listener endpoint annotations

To enable support for @JmsListener annotations add @EnableJms to one of
your @Configuration classes.

By default, the infrastructure looks for a bean named jmsListenerContainerFactory
as the source for the factory to use to create message listener containers. In this
case, and ignoring the JMS infrastructure setup, the processOrder method can be
invoked with a core poll size of 3 threads and a maximum pool size of 10 threads.

It is possible to customize the listener container factory to use per annotation or
an explicit default can be configured by implementing the JmsListenerConfigurer
interface. The default is only required if at least one endpoint is registered
without a specific container factory. See the javadoc for full details and examples.

3.6.2. Programmatic endpoints registration

JmsListenerEndpoint provides a model of an JMS endpoint and is responsible for configuring
the container for that model. The infrastructure allows you to configure endpoints
programmatically in addition to the ones that are detected by the JmsListener annotation.

In the example above, we used SimpleJmsListenerEndpoint which provides the actual
MessageListener to invoke but you could just as well build your own endpoint variant
describing a custom invocation mechanism.

It should be noted that you could just as well skip the use of @JmsListener altogether
and only register your endpoints programmatically through JmsListenerConfigurer.

3.6.3. Annotated endpoint method signature

So far, we have been injecting a simple String in our endpoint but it can actually
have a very flexible method signature. Let’s rewrite it to inject the Order with
a custom header:

@Headers-annotated argument that must also be assignable to java.util.Map for
getting access to all headers.

A non-annotated element that is not one of the supported types (i.e. Message and
Session) is considered to be the payload. You can make that explicit by annotating
the parameter with @Payload. You can also turn on validation by adding an extra
@Valid.

The ability to inject Spring’s Message abstraction is particularly useful to benefit
from all the information stored in the transport-specific message without relying on
transport-specific API.

Handling of method arguments is provided by DefaultMessageHandlerMethodFactory which can be
further customized to support additional method arguments. The conversion and validation
support can be customized there as well.

For instance, if we want to make sure our Order is valid before processing it, we can
annotate the payload with @Valid and configure the necessary validator as follows:

3.6.4. Response management

The existing support in MessageListenerAdapter
already allows your method to have a non-void return type. When that’s the case, the result of
the invocation is encapsulated in a javax.jms.Message sent either in the destination specified
in the JMSReplyTo header of the original message or in the default destination configured on
the listener. That default destination can now be set using the @SendTo annotation of the
messaging abstraction.

Assuming our processOrder method should now return an OrderStatus, it is possible to write it
as follow to automatically send a response:

If you need to compute the response destination at runtime, you can encapsulate your response
in a JmsResponse instance that also provides the destination to use at runtime. The previous
example can be rewritten as follows:

The namespace consists of three top-level elements: <annotation-driven/>, <listener-container/>
and <jca-listener-container/>. <annotation-driven enables the use of annotation-driven listener endpoints. <listener-container/> and <jca-listener-container/>
defines shared listener container configuration and may contain <listener/> child elements. Here
is an example of a basic configuration for two listeners.

The example above is equivalent to creating two distinct listener container bean
definitions and two distinct MessageListenerAdapter bean definitions as demonstrated
in MessageListenerAdapter. In addition to the attributes shown
above, the listener element may contain several optional ones. The following table
describes all available attributes:

Table 3. Attributes of the JMS <listener> element

Attribute

Description

id

A bean name for the hosting listener container. If not specified, a bean name will be
automatically generated.

destination (required)

The destination name for this listener, resolved through the DestinationResolver
strategy.

ref (required)

The bean name of the handler object.

method

The name of the handler method to invoke. If the ref points to a MessageListener
or Spring SessionAwareMessageListener, this attribute may be omitted.

response-destination

The name of the default response destination to send response messages to. This will
be applied in case of a request message that does not carry a "JMSReplyTo" field. The
type of this destination will be determined by the listener-container’s
"response-destination-type" attribute. Note: This only applies to a listener method with a
return value, for which each result object will be converted into a response message.

subscription

The name of the durable subscription, if any.

selector

An optional message selector for this listener.

concurrency

The number of concurrent sessions/consumers to start for this listener. Can either be
a simple number indicating the maximum number (e.g. "5") or a range indicating the lower
as well as the upper limit (e.g. "3-5"). Note that a specified minimum is just a hint
and might be ignored at runtime. Default is the value provided by the container

The <listener-container/> element also accepts several optional attributes. This
allows for customization of the various strategies (for example, taskExecutor and
destinationResolver) as well as basic JMS settings and resource references. Using
these attributes, it is possible to define highly-customized listener containers while
still benefiting from the convenience of the namespace.

Such settings can be automatically exposed as a JmsListenerContainerFactory by
specifying the id of the bean to expose through the factory-id attribute.

The following table describes all available attributes. Consult the class-level javadocs
of the AbstractMessageListenerContainer and its concrete subclasses for more details
on the individual properties. The javadocs also provide a discussion of transaction
choices and message redelivery scenarios.

Table 4. Attributes of the JMS <listener-container> element

Attribute

Description

container-type

The type of this listener container. Available options are: default, simple,
default102, or simple102 (the default value is 'default').

container-class

A custom listener container implementation class as fully qualified class name.
Default is Spring’s standard DefaultMessageListenerContainer or
SimpleMessageListenerContainer, according to the "container-type" attribute.

factory-id

Exposes the settings defined by this element as a JmsListenerContainerFactory
with the specified id so that they can be reused with other endpoints.

connection-factory

A reference to the JMS ConnectionFactory bean (the default bean name is
'connectionFactory').

task-executor

A reference to the Spring TaskExecutor for the JMS listener invokers.

destination-resolver

A reference to the DestinationResolver strategy for resolving JMS Destinations.

message-converter

A reference to the MessageConverter strategy for converting JMS Messages to listener
method arguments. Default is a SimpleMessageConverter.

error-handler

A reference to an ErrorHandler strategy for handling any uncaught Exceptions that
may occur during the execution of the MessageListener.

destination-type

The JMS destination type for this listener: queue, topic, durableTopic, sharedTopic
or sharedDurableTopic. This enables potentially the pubSubDomain, subscriptionDurable
and subscriptionShared properties of the container. The default is queue (i.e. disabling
those 3 properties).

response-destination-type

The JMS destination type for responses: "queue", "topic". Default is the value of the
"destination-type" attribute.

client-id

The JMS client id for this listener container. Needs to be specified when using
durable subscriptions.

cache

The cache level for JMS resources: none, connection, session, consumer or
auto. By default ( auto), the cache level will effectively be "consumer", unless
an external transaction manager has been specified - in which case the effective
default will be none (assuming Java EE-style transaction management where the given
ConnectionFactory is an XA-aware pool).

acknowledge

The native JMS acknowledge mode: auto, client, dups-ok or transacted. A value
of transacted activates a locally transacted Session. As an alternative, specify
the transaction-manager attribute described below. Default is auto.

transaction-manager

A reference to an external PlatformTransactionManager (typically an XA-based
transaction coordinator, e.g. Spring’s JtaTransactionManager). If not specified,
native acknowledging will be used (see "acknowledge" attribute).

concurrency

The number of concurrent sessions/consumers to start for each listener. Can either be
a simple number indicating the maximum number (e.g. "5") or a range indicating the
lower as well as the upper limit (e.g. "3-5"). Note that a specified minimum is just a
hint and might be ignored at runtime. Default is 1; keep concurrency limited to 1 in
case of a topic listener or if queue ordering is important; consider raising it for
general queues.

prefetch

The maximum number of messages to load into a single session. Note that raising this
number might lead to starvation of concurrent consumers!

receive-timeout

The timeout to use for receive calls (in milliseconds). The default is 1000 ms (1
sec); -1 indicates no timeout at all.

back-off

Specify the BackOff instance to use to compute the interval between recovery
attempts. If the BackOffExecution implementation returns BackOffExecution#STOP,
the listener container will not further attempt to recover. The recovery-interval
value is ignored when this property is set. The default is a FixedBackOff with
an interval of 5000 ms, that is 5 seconds.

recovery-interval

Specify the interval between recovery attempts, in milliseconds. Convenience
way to create a FixedBackOff with the specified interval. For more recovery
options, consider specifying a BackOff instance instead. The default is 5000 ms,
that is 5 seconds.

phase

The lifecycle phase within which this container should start and stop. The lower the
value the earlier this container will start and the later it will stop. The default is
Integer.MAX_VALUE meaning the container will start as late as possible and stop as
soon as possible.

Configuring a JCA-based listener container with the "jms" schema support is very similar.

The available configuration options for the JCA variant are described in the following
table:

Table 5. Attributes of the JMS <jca-listener-container/> element

Attribute

Description

factory-id

Exposes the settings defined by this element as a JmsListenerContainerFactory
with the specified id so that they can be reused with other endpoints.

resource-adapter

A reference to the JCA ResourceAdapter bean (the default bean name is
'resourceAdapter').

activation-spec-factory

A reference to the JmsActivationSpecFactory. The default is to autodetect the JMS
provider and its ActivationSpec class (see DefaultJmsActivationSpecFactory)

destination-resolver

A reference to the DestinationResolver strategy for resolving JMS Destinations.

message-converter

A reference to the MessageConverter strategy for converting JMS Messages to listener
method arguments. Default is a SimpleMessageConverter.

destination-type

The JMS destination type for this listener: queue, topic, durableTopic, sharedTopic
or sharedDurableTopic. This enables potentially the pubSubDomain, subscriptionDurable
and subscriptionShared properties of the container. The default is queue (i.e. disabling
those 3 properties).

response-destination-type

The JMS destination type for responses: "queue", "topic". Default is the value of the
"destination-type" attribute.

client-id

The JMS client id for this listener container. Needs to be specified when using
durable subscriptions.

acknowledge

The native JMS acknowledge mode: auto, client, dups-ok or transacted. A value
of transacted activates a locally transacted Session. As an alternative, specify
the transaction-manager attribute described below. Default is auto.

transaction-manager

A reference to a Spring JtaTransactionManager or a
javax.transaction.TransactionManager for kicking off an XA transaction for each
incoming message. If not specified, native acknowledging will be used (see the
"acknowledge" attribute).

concurrency

The number of concurrent sessions/consumers to start for each listener. Can either be
a simple number indicating the maximum number (e.g. "5") or a range indicating the
lower as well as the upper limit (e.g. "3-5"). Note that a specified minimum is just a
hint and will typically be ignored at runtime when using a JCA listener container.
Default is 1.

prefetch

The maximum number of messages to load into a single session. Note that raising this
number might lead to starvation of concurrent consumers!

4. JMX

4.1. Introduction

The JMX support in Spring provides you with the features to easily and transparently
integrate your Spring application into a JMX infrastructure.

JMX?

This chapter is not an introduction to JMX…​ it doesn’t try to explain the motivations
of why one might want to use JMX (or indeed what the letters JMX actually stand for). If
you are new to JMX, check out Further resources at the end of this chapter.

Specifically, Spring’s JMX support provides four core features:

The automatic registration of any Spring bean as a JMX MBean

A flexible mechanism for controlling the management interface of your beans

The declarative exposure of MBeans over remote, JSR-160 connectors

The simple proxying of both local and remote MBean resources

These features are designed to work without coupling your application components to
either Spring or JMX interfaces and classes. Indeed, for the most part your application
classes need not be aware of either Spring or JMX in order to take advantage of the
Spring JMX features.

4.2. Exporting your beans to JMX

The core class in Spring’s JMX framework is the MBeanExporter. This class is
responsible for taking your Spring beans and registering them with a JMX MBeanServer.
For example, consider the following class:

To expose the properties and methods of this bean as attributes and operations of an
MBean you simply configure an instance of the MBeanExporter class in your
configuration file and pass in the bean as shown below:

<beans><!-- this bean must not be lazily initialized if the exporting is to happen --><beanid="exporter"class="org.springframework.jmx.export.MBeanExporter"lazy-init="false"><propertyname="beans"><map><entrykey="bean:name=testBean1"value-ref="testBean"/></map></property></bean><beanid="testBean"class="org.springframework.jmx.JmxTestBean"><propertyname="name"value="TEST"/><propertyname="age"value="100"/></bean></beans>

The pertinent bean definition from the above configuration snippet is the exporter
bean. The beans property tells the MBeanExporter exactly which of your beans must be
exported to the JMX MBeanServer. In the default configuration, the key of each entry
in the beansMap is used as the ObjectName for the bean referenced by the
corresponding entry value. This behavior can be changed as described in Controlling the ObjectNames for your beans.

With this configuration the testBean bean is exposed as an MBean under the
ObjectNamebean:name=testBean1. By default, all public properties of the bean
are exposed as attributes and all public methods (bar those inherited from the
Object class) are exposed as operations.

MBeanExporter is a Lifecycle bean (see Startup and shutdown callbacks) and MBeans are exported as late as possible during
the application lifecycle by default. It is possible to configure the phase at which
the export happens or disable automatic registration by setting the autoStartup flag.

4.2.1. Creating an MBeanServer

The above configuration assumes that the application is running in an environment that
has one (and only one) MBeanServer already running. In this case, Spring will attempt
to locate the running MBeanServer and register your beans with that server (if any).
This behavior is useful when your application is running inside a container such as
Tomcat or IBM WebSphere that has its own MBeanServer.

However, this approach is of no use in a standalone environment, or when running inside
a container that does not provide an MBeanServer. To address this you can create an
MBeanServer instance declaratively by adding an instance of the
org.springframework.jmx.support.MBeanServerFactoryBean class to your configuration.
You can also ensure that a specific MBeanServer is used by setting the value of the
MBeanExporter's server property to the MBeanServer value returned by an
MBeanServerFactoryBean; for example:

<beans><beanid="mbeanServer"class="org.springframework.jmx.support.MBeanServerFactoryBean"/><!--
this bean needs to be eagerly pre-instantiated in order for the exporting to occur;
this means that it must not be marked as lazily initialized
--><beanid="exporter"class="org.springframework.jmx.export.MBeanExporter"><propertyname="beans"><map><entrykey="bean:name=testBean1"value-ref="testBean"/></map></property><propertyname="server"ref="mbeanServer"/></bean><beanid="testBean"class="org.springframework.jmx.JmxTestBean"><propertyname="name"value="TEST"/><propertyname="age"value="100"/></bean></beans>

Here an instance of MBeanServer is created by the MBeanServerFactoryBean and is
supplied to the MBeanExporter via the server property. When you supply your own
MBeanServer instance, the MBeanExporter will not attempt to locate a running
MBeanServer and will use the supplied MBeanServer instance. For this to work
correctly, you must (of course) have a JMX implementation on your classpath.

4.2.2. Reusing an existing MBeanServer

If no server is specified, the MBeanExporter tries to automatically detect a running
MBeanServer. This works in most environment where only one MBeanServer instance is
used, however when multiple instances exist, the exporter might pick the wrong server.
In such cases, one should use the MBeanServeragentId to indicate which instance to
be used:

<beans><beanid="mbeanServer"class="org.springframework.jmx.support.MBeanServerFactoryBean"><!-- indicate to first look for a server --><propertyname="locateExistingServerIfPossible"value="true"/><!-- search for the MBeanServer instance with the given agentId --><propertyname="agentId"value="MBeanServer_instance_agentId>"/></bean><beanid="exporter"class="org.springframework.jmx.export.MBeanExporter"><propertyname="server"ref="mbeanServer"/>
...
</bean></beans>

For platforms/cases where the existing MBeanServer has a dynamic (or unknown)
agentId which is retrieved through lookup methods, one should use
factory-method:

4.2.3. Lazy-initialized MBeans

If you configure a bean with the MBeanExporter that is also configured for lazy
initialization, then the MBeanExporter will not break this contract and will avoid
instantiating the bean. Instead, it will register a proxy with the MBeanServer and
will defer obtaining the bean from the container until the first invocation on the proxy
occurs.

4.2.4. Automatic registration of MBeans

Any beans that are exported through the MBeanExporter and are already valid MBeans are
registered as-is with the MBeanServer without further intervention from Spring. MBeans
can be automatically detected by the MBeanExporter by setting the autodetect
property to true:

Here, the bean called spring:mbean=true is already a valid JMX MBean and will be
automatically registered by Spring. By default, beans that are autodetected for JMX
registration have their bean name used as the ObjectName. This behavior can be
overridden as detailed in Controlling the ObjectNames for your beans.

4.2.5. Controlling the registration behavior

Consider the scenario where a Spring MBeanExporter attempts to register an MBean
with an MBeanServer using the ObjectName'bean:name=testBean1'. If an MBean
instance has already been registered under that same ObjectName, the default behavior
is to fail (and throw an InstanceAlreadyExistsException).

It is possible to control the behavior of exactly what happens when an MBean is
registered with an MBeanServer. Spring’s JMX support allows for three different
registration behaviors to control the registration behavior when the registration
process finds that an MBean has already been registered under the same ObjectName;
these registration behaviors are summarized on the following table:

Table 6. Registration Behaviors

Registration behavior

Explanation

REGISTRATION_FAIL_ON_EXISTING

This is the default registration behavior. If an MBean instance has already been
registered under the same ObjectName, the MBean that is being registered will not
be registered and an InstanceAlreadyExistsException will be thrown. The existing
MBean is unaffected.

REGISTRATION_IGNORE_EXISTING

If an MBean instance has already been registered under the same ObjectName, the
MBean that is being registered will not be registered. The existing MBean is
unaffected, and no Exception will be thrown. This is useful in settings where
multiple applications want to share a common MBean in a shared MBeanServer.

REGISTRATION_REPLACE_EXISTING

If an MBean instance has already been registered under the same ObjectName, the
existing MBean that was previously registered will be unregistered and the new
MBean will be registered in its place (the new MBean effectively replaces the
previous instance).

The above values are defined as constants on the MBeanRegistrationSupport class (the
MBeanExporter class derives from this superclass). If you want to change the default
registration behavior, you simply need to set the value of the
registrationBehaviorName property on your MBeanExporter definition to one of those
values.

The following example illustrates how to effect a change from the default registration
behavior to the REGISTRATION_REPLACE_EXISTING behavior:

4.3. Controlling the management interface of your beans

In the previous example, you had little control over the management interface of your
bean; all of the public properties and methods of each exported bean was exposed
as JMX attributes and operations respectively. To exercise finer-grained control over
exactly which properties and methods of your exported beans are actually exposed as JMX
attributes and operations, Spring JMX provides a comprehensive and extensible mechanism
for controlling the management interfaces of your beans.

4.3.1. MBeanInfoAssembler interface

Behind the scenes, the MBeanExporter delegates to an implementation of the
org.springframework.jmx.export.assembler.MBeanInfoAssembler interface which is
responsible for defining the management interface of each bean that is being exposed.
The default implementation,
org.springframework.jmx.export.assembler.SimpleReflectiveMBeanInfoAssembler, simply
defines a management interface that exposes all public properties and methods (as you
saw in the previous examples). Spring provides two additional implementations of the
MBeanInfoAssembler interface that allow you to control the generated management
interface using either source-level metadata or any arbitrary interface.

4.3.2. Using source-level metadata: Java annotations

Using the MetadataMBeanInfoAssembler you can define the management interfaces for your
beans using source level metadata. The reading of metadata is encapsulated by the
org.springframework.jmx.export.metadata.JmxAttributeSource interface. Spring JMX
provides a default implementation which uses Java annotations, namely
org.springframework.jmx.export.annotation.AnnotationJmxAttributeSource. The
MetadataMBeanInfoAssemblermust be configured with an implementation instance of
the JmxAttributeSource interface for it to function correctly (there is no
default).

To mark a bean for export to JMX, you should annotate the bean class with the
ManagedResource annotation. Each method you wish to expose as an operation must be
marked with the ManagedOperation annotation and each property you wish to expose must
be marked with the ManagedAttribute annotation. When marking properties you can omit
either the annotation of the getter or the setter to create a write-only or read-only
attribute respectively.

A ManagedResource annotated bean must be public as well as the methods exposing
an operation or an attribute.

The example below shows the annotated version of the JmxTestBean class that you saw
earlier:

Here you can see that the JmxTestBean class is marked with the ManagedResource
annotation and that this ManagedResource annotation is configured with a set of
properties. These properties can be used to configure various aspects of the MBean that
is generated by the MBeanExporter, and are explained in greater detail later in
section entitled Source-level metadata types.

You will also notice that both the age and name properties are annotated with the
ManagedAttribute annotation, but in the case of the age property, only the getter is
marked. This will cause both of these properties to be included in the management
interface as attributes, but the age attribute will be read-only.

Finally, you will notice that the add(int, int) method is marked with the
ManagedOperation attribute whereas the dontExposeMe() method is not. This will cause
the management interface to contain only one operation, add(int, int), when using the
MetadataMBeanInfoAssembler.

The configuration below shows how you configure the MBeanExporter to use the
MetadataMBeanInfoAssembler:

<beans><beanid="exporter"class="org.springframework.jmx.export.MBeanExporter"><propertyname="assembler"ref="assembler"/><propertyname="namingStrategy"ref="namingStrategy"/><propertyname="autodetect"value="true"/></bean><beanid="jmxAttributeSource"class="org.springframework.jmx.export.annotation.AnnotationJmxAttributeSource"/><!-- will create management interface using annotation metadata --><beanid="assembler"class="org.springframework.jmx.export.assembler.MetadataMBeanInfoAssembler"><propertyname="attributeSource"ref="jmxAttributeSource"/></bean><!-- will pick up the ObjectName from the annotation --><beanid="namingStrategy"class="org.springframework.jmx.export.naming.MetadataNamingStrategy"><propertyname="attributeSource"ref="jmxAttributeSource"/></bean><beanid="testBean"class="org.springframework.jmx.AnnotationTestBean"><propertyname="name"value="TEST"/><propertyname="age"value="100"/></bean></beans>

Here you can see that an MetadataMBeanInfoAssembler bean has been configured with an
instance of the AnnotationJmxAttributeSource class and passed to the MBeanExporter
through the assembler property. This is all that is required to take advantage of
metadata-driven management interfaces for your Spring-exposed MBeans.

4.3.3. Source-level metadata types

The following source level metadata types are available for use in Spring JMX:

Table 7. Source-level metadata types

Purpose

Annotation

Annotation Type

Mark all instances of a Class as JMX managed resources

@ManagedResource

Class

Mark a method as a JMX operation

@ManagedOperation

Method

Mark a getter or setter as one half of a JMX attribute

@ManagedAttribute

Method (only getters and setters)

Define descriptions for operation parameters

@ManagedOperationParameter and @ManagedOperationParameters

Method

The following configuration parameters are available for use on these source-level
metadata types:

Table 8. Source-level metadata parameters

Parameter

Description

Applies to

ObjectName

Used by MetadataNamingStrategy to determine the ObjectName of a managed resource

4.3.4. AutodetectCapableMBeanInfoAssembler interface

To simplify configuration even further, Spring introduces the
AutodetectCapableMBeanInfoAssembler interface which extends the MBeanInfoAssembler
interface to add support for autodetection of MBean resources. If you configure the
MBeanExporter with an instance of AutodetectCapableMBeanInfoAssembler then it is
allowed to "vote" on the inclusion of beans for exposure to JMX.

Out of the box, the only implementation of the AutodetectCapableMBeanInfo interface is
the MetadataMBeanInfoAssembler which will vote to include any bean which is marked
with the ManagedResource attribute. The default approach in this case is to use the
bean name as the ObjectName which results in a configuration like this:

<beans><beanid="exporter"class="org.springframework.jmx.export.MBeanExporter"><!-- notice how no 'beans' are explicitly configured here --><propertyname="autodetect"value="true"/><propertyname="assembler"ref="assembler"/></bean><beanid="testBean"class="org.springframework.jmx.JmxTestBean"><propertyname="name"value="TEST"/><propertyname="age"value="100"/></bean><beanid="assembler"class="org.springframework.jmx.export.assembler.MetadataMBeanInfoAssembler"><propertyname="attributeSource"><beanclass="org.springframework.jmx.export.annotation.AnnotationJmxAttributeSource"/></property></bean></beans>

Notice that in this configuration no beans are passed to the MBeanExporter; however,
the JmxTestBean will still be registered since it is marked with the ManagedResource
attribute and the MetadataMBeanInfoAssembler detects this and votes to include it. The
only problem with this approach is that the name of the JmxTestBean now has business
meaning. You can address this issue by changing the default behavior for ObjectName
creation as defined in Controlling the ObjectNames for your beans.

4.3.5. Defining management interfaces using Java interfaces

In addition to the MetadataMBeanInfoAssembler, Spring also includes the
InterfaceBasedMBeanInfoAssembler which allows you to constrain the methods and
properties that are exposed based on the set of methods defined in a collection of
interfaces.

Although the standard mechanism for exposing MBeans is to use interfaces and a simple
naming scheme, the InterfaceBasedMBeanInfoAssembler extends this functionality by
removing the need for naming conventions, allowing you to use more than one interface
and removing the need for your beans to implement the MBean interfaces.

Consider this interface that is used to define a management interface for the
JmxTestBean class that you saw earlier:

This interface defines the methods and properties that will be exposed as operations and
attributes on the JMX MBean. The code below shows how to configure Spring JMX to use
this interface as the definition for the management interface:

Here you can see that the InterfaceBasedMBeanInfoAssembler is configured to use the
IJmxTestBean interface when constructing the management interface for any bean. It is
important to understand that beans processed by the InterfaceBasedMBeanInfoAssembler
are not required to implement the interface used to generate the JMX management
interface.

In the case above, the IJmxTestBean interface is used to construct all management
interfaces for all beans. In many cases this is not the desired behavior and you may
want to use different interfaces for different beans. In this case, you can pass
InterfaceBasedMBeanInfoAssembler a Properties instance via the interfaceMappings
property, where the key of each entry is the bean name and the value of each entry is a
comma-separated list of interface names to use for that bean.

If no management interface is specified through either the managedInterfaces or
interfaceMappings properties, then the InterfaceBasedMBeanInfoAssembler will reflect
on the bean and use all of the interfaces implemented by that bean to create the
management interface.

4.3.6. Using MethodNameBasedMBeanInfoAssembler

The MethodNameBasedMBeanInfoAssembler allows you to specify a list of method names
that will be exposed to JMX as attributes and operations. The code below shows a sample
configuration for this:

Here you can see that the methods add and myOperation will be exposed as JMX
operations and getName(), setName(String) and getAge() will be exposed as the
appropriate half of a JMX attribute. In the code above, the method mappings apply to
beans that are exposed to JMX. To control method exposure on a bean-by-bean basis, use
the methodMappings property of MethodNameMBeanInfoAssembler to map bean names to
lists of method names.

4.4. Controlling the ObjectNames for your beans

Behind the scenes, the MBeanExporter delegates to an implementation of the
ObjectNamingStrategy to obtain ObjectNames for each of the beans it is registering.
The default implementation, KeyNamingStrategy, will, by default, use the key of the
beansMap as the ObjectName. In addition, the KeyNamingStrategy can map the key
of the beansMap to an entry in a Properties file (or files) to resolve the
ObjectName. In addition to the KeyNamingStrategy, Spring provides two additional
ObjectNamingStrategy implementations: the IdentityNamingStrategy that builds an
ObjectName based on the JVM identity of the bean and the MetadataNamingStrategy that
uses source level metadata to obtain the ObjectName.

4.4.1. Reading ObjectNames from Properties

You can configure your own KeyNamingStrategy instance and configure it to read
ObjectNames from a Properties instance rather than use bean key. The
KeyNamingStrategy will attempt to locate an entry in the Properties with a key
corresponding to the bean key. If no entry is found or if the Properties instance is
null then the bean key itself is used.

The code below shows a sample configuration for the KeyNamingStrategy:

Here an instance of KeyNamingStrategy is configured with a Properties instance that
is merged from the Properties instance defined by the mapping property and the
properties files located in the paths defined by the mappings property. In this
configuration, the testBean bean will be given the ObjectNamebean:name=testBean1
since this is the entry in the Properties instance that has a key corresponding to the
bean key.

If no entry in the Properties instance can be found then the bean key name is used as
the ObjectName.

4.4.2. Using the MetadataNamingStrategy

The MetadataNamingStrategy uses the objectName property of the ManagedResource
attribute on each bean to create the ObjectName. The code below shows the
configuration for the MetadataNamingStrategy:

If no objectName has been provided for the ManagedResource attribute, then an
ObjectName will be created with the following
format:[fully-qualified-package-name]:type=[short-classname],name=[bean-name]. For
example, the generated ObjectName for the following bean would be:
com.foo:type=MyClass,name=myBean.

<beanid="myBean"class="com.foo.MyClass"/>

4.4.3. Configuring annotation based MBean export

If you prefer using the annotation based approach to define
your management interfaces, then a convenience subclass of MBeanExporter is available:
AnnotationMBeanExporter. When defining an instance of this subclass, the
namingStrategy, assembler, and attributeSource configuration is no longer needed,
since it will always use standard Java annotation-based metadata (autodetection is
always enabled as well). In fact, rather than defining an MBeanExporter bean, an even
simpler syntax is supported by the @EnableMBeanExport@Configuration annotation.

@Configuration@EnableMBeanExportpublicclassAppConfig {
}

If you prefer XML based configuration the 'context:mbean-export' element serves the
same purpose.

<context:mbean-export/>

You can provide a reference to a particular MBean server if necessary, and the
defaultDomain attribute (a property of AnnotationMBeanExporter) accepts an alternate
value for the generated MBean `ObjectNames’ domains. This would be used in place of the
fully qualified package name as described in the previous section on
MetadataNamingStrategy.

Do not use interface-based AOP proxies in combination with autodetection of JMX
annotations in your bean classes. Interface-based proxies 'hide' the target class, which
also hides the JMX managed resource annotations. Hence, use target-class proxies in that
case: through setting the 'proxy-target-class' flag on <aop:config/>,
<tx:annotation-driven/>, etc. Otherwise, your JMX beans might be silently ignored at
startup…​

4.5.1. Server-side connectors

By default ConnectorServerFactoryBean creates a JMXConnectorServer bound to
"service:jmx:jmxmp://localhost:9875". The serverConnector bean thus exposes the
local MBeanServer to clients through the JMXMP protocol on localhost, port 9875. Note
that the JMXMP protocol is marked as optional by the JSR 160 specification: currently,
the main open-source JMX implementation, MX4J, and the one provided with the JDK
do not support JMXMP.

To specify another URL and register the JMXConnectorServer itself with the
MBeanServer use the serviceUrl and ObjectName properties respectively:

If the ObjectName property is set Spring will automatically register your connector
with the MBeanServer under that ObjectName. The example below shows the full set of
parameters which you can pass to the ConnectorServerFactoryBean when creating a
JMXConnector:

Note that when using a RMI-based connector you need the lookup service (tnameserv or
rmiregistry) to be started in order for the name registration to complete. If you are
using Spring to export remote services for you via RMI, then Spring will already have
constructed an RMI registry. If not, you can easily start a registry using the following
snippet of configuration:

4.5.3. JMX over Hessian or SOAP

JSR-160 permits extensions to the way in which communication is done between the client
and the server. The examples above are using the mandatory RMI-based implementation
required by the JSR-160 specification (IIOP and JRMP) and the (optional) JMXMP. By using
other providers or JMX implementations (such as MX4J) you
can take advantage of protocols like SOAP or Hessian over simple HTTP or SSL and others:

In the case of the above example, MX4J 3.0.0 was used; see the official MX4J
documentation for more information.

4.6. Accessing MBeans via proxies

Spring JMX allows you to create proxies that re-route calls to MBeans registered in a
local or remote MBeanServer. These proxies provide you with a standard Java interface
through which you can interact with your MBeans. The code below shows how to configure a
proxy for an MBean running in a local MBeanServer:

Here you can see that a proxy is created for the MBean registered under the
ObjectName: bean:name=testBean. The set of interfaces that the proxy will implement
is controlled by the proxyInterfaces property and the rules for mapping methods and
properties on these interfaces to operations and attributes on the MBean are the same
rules used by the InterfaceBasedMBeanInfoAssembler.

The MBeanProxyFactoryBean can create a proxy to any MBean that is accessible via an
MBeanServerConnection. By default, the local MBeanServer is located and used, but
you can override this and provide an MBeanServerConnection pointing to a remote
MBeanServer to cater for proxies pointing to remote MBeans:

Here you can see that we create an MBeanServerConnection pointing to a remote machine
using the MBeanServerConnectionFactoryBean. This MBeanServerConnection is then
passed to the MBeanProxyFactoryBean via the server property. The proxy that is
created will forward all invocations to the MBeanServer via this
MBeanServerConnection.

4.7. Notifications

Spring’s JMX offering includes comprehensive support for JMX notifications.

4.7.1. Registering listeners for notifications

Spring’s JMX support makes it very easy to register any number of
NotificationListeners with any number of MBeans (this includes MBeans exported by
Spring’s MBeanExporter and MBeans registered via some other mechanism). By way of an
example, consider the scenario where one would like to be informed (via a
Notification) each and every time an attribute of a target MBean changes.

With the above configuration in place, every time a JMX Notification is broadcast from
the target MBean ( bean:name=testBean1), the ConsoleLoggingNotificationListener bean
that was registered as a listener via the notificationListenerMappings property will
be notified. The ConsoleLoggingNotificationListener bean can then take whatever action
it deems appropriate in response to the Notification.

You can also use straight bean names as the link between exported beans and listeners:

If one wants to register a single NotificationListener instance for all of the beans
that the enclosing MBeanExporter is exporting, one can use the special wildcard '*'
(sans quotes) as the key for an entry in the notificationListenerMappings property
map; for example:

If one needs to do the inverse (that is, register a number of distinct listeners against
an MBean), then one has to use the notificationListeners list property instead (and in
preference to the notificationListenerMappings property). This time, instead of
configuring simply a NotificationListener for a single MBean, one configures
NotificationListenerBean instances…​ a NotificationListenerBean encapsulates a
NotificationListener and the ObjectName (or ObjectNames) that it is to be
registered against in an MBeanServer. The NotificationListenerBean also encapsulates
a number of other properties such as a NotificationFilter and an arbitrary handback
object that can be used in advanced JMX notification scenarios.

The configuration when using NotificationListenerBean instances is not wildly
different to what was presented previously:

The above example is equivalent to the first notification example. Lets assume then that
we want to be given a handback object every time a Notification is raised, and that
additionally we want to filter out extraneous Notifications by supplying a
NotificationFilter. (For a full discussion of just what a handback object is, and
indeed what a NotificationFilter is, please do consult that section of the JMX
specification (1.2) entitled 'The JMX Notification Model'.)

<beans><beanid="exporter"class="org.springframework.jmx.export.MBeanExporter"><propertyname="beans"><map><entrykey="bean:name=testBean1"value-ref="testBean1"/><entrykey="bean:name=testBean2"value-ref="testBean2"/></map></property><propertyname="notificationListeners"><list><beanclass="org.springframework.jmx.export.NotificationListenerBean"><constructor-argref="customerNotificationListener"/><propertyname="mappedObjectNames"><list><!-- handles notifications from two distinct MBeans --><value>bean:name=testBean1</value><value>bean:name=testBean2</value></list></property><propertyname="handback"><beanclass="java.lang.String"><constructor-argvalue="This could be anything..."/></bean></property><propertyname="notificationFilter"ref="customerNotificationListener"/></bean></list></property></bean><!-- implements both the NotificationListener and NotificationFilter interfaces --><beanid="customerNotificationListener"class="com.example.ConsoleLoggingNotificationListener"/><beanid="testBean1"class="org.springframework.jmx.JmxTestBean"><propertyname="name"value="TEST"/><propertyname="age"value="100"/></bean><beanid="testBean2"class="org.springframework.jmx.JmxTestBean"><propertyname="name"value="ANOTHER TEST"/><propertyname="age"value="200"/></bean></beans>

4.7.2. Publishing Notifications

Spring provides support not just for registering to receive Notifications, but also
for publishing Notifications.

Please note that this section is really only relevant to Spring managed beans that have
been exposed as MBeans via an MBeanExporter; any existing, user-defined MBeans should
use the standard JMX APIs for notification publication.

The key interface in Spring’s JMX notification publication support is the
NotificationPublisher interface (defined in the
org.springframework.jmx.export.notification package). Any bean that is going to be
exported as an MBean via an MBeanExporter instance can implement the related
NotificationPublisherAware interface to gain access to a NotificationPublisher
instance. The NotificationPublisherAware interface simply supplies an instance of a
NotificationPublisher to the implementing bean via a simple setter method, which the
bean can then use to publish Notifications.

As stated in the javadocs of the NotificationPublisher class, managed beans that are
publishing events via the NotificationPublisher mechanism are not responsible for
the state management of any notification listeners and the like …​ Spring’s JMX support
will take care of handling all the JMX infrastructure issues. All one need do as an
application developer is implement the NotificationPublisherAware interface and start
publishing events using the supplied NotificationPublisher instance. Note that the
NotificationPublisher will be set after the managed bean has been registered with
an MBeanServer.

Using a NotificationPublisher instance is quite straightforward…​ one simply creates
a JMX Notification instance (or an instance of an appropriate Notification
subclass), populates the notification with the data pertinent to the event that is to be
published, and one then invokes the sendNotification(Notification) on the
NotificationPublisher instance, passing in the Notification.

Find below a simple example…​ in this scenario, exported instances of the JmxTestBean
are going to publish a NotificationEvent every time the add(int, int) operation is
invoked.

The NotificationPublisher interface and the machinery to get it all working is one of
the nicer features of Spring’s JMX support. It does however come with the price tag of
coupling your classes to both Spring and JMX; as always, the advice here is to be
pragmatic…​ if you need the functionality offered by the NotificationPublisher and
you can accept the coupling to both Spring and JMX, then do so.

4.8. Further resources

The MX4J homepage (an Open Source implementation of
various JMX specs)

5. JCA CCI

5.1. Introduction

Java EE provides a specification to standardize access to enterprise information systems
(EIS): the JCA (Java EE Connector Architecture). This specification is divided into
several different parts:

SPI (Service provider interfaces) that the connector provider must implement. These
interfaces constitute a resource adapter which can be deployed on a Java EE
application server. In such a scenario, the server manages connection pooling,
transaction and security (managed mode). The application server is also responsible
for managing the configuration, which is held outside the client application. A
connector can be used without an application server as well; in this case, the
application must configure it directly (non-managed mode).

CCI (Common Client Interface) that an application can use to interact with the
connector and thus communicate with an EIS. An API for local transaction demarcation
is provided as well.

The aim of the Spring CCI support is to provide classes to access a CCI connector in
typical Spring style, leveraging the Spring Framework’s general resource and transaction
management facilities.

The client side of connectors doesn’t alway use CCI. Some connectors expose their own
APIs, only providing JCA resource adapter to use the system contracts of a Java EE
container (connection pooling, global transactions, security). Spring does not offer
special support for such connector-specific APIs.

5.2. Configuring CCI

5.2.1. Connector configuration

The base resource to use JCA CCI is the ConnectionFactory interface. The connector
used must provide an implementation of this interface.

To use your connector, you can deploy it on your application server and fetch the
ConnectionFactory from the server’s JNDI environment (managed mode). The connector
must be packaged as a RAR file (resource adapter archive) and contain a ra.xml file to
describe its deployment characteristics. The actual name of the resource is specified
when you deploy it. To access it within Spring, simply use Spring’s
JndiObjectFactoryBean / <jee:jndi-lookup> fetch the factory by its JNDI name.

Another way to use a connector is to embed it in your application (non-managed mode),
not using an application server to deploy and configure it. Spring offers the
possibility to configure a connector as a bean, through a provided FactoryBean (
LocalConnectionFactoryBean). In this manner, you only need the connector library in
the classpath (no RAR file and no ra.xml descriptor needed). The library must be
extracted from the connector’s RAR file, if necessary.

Once you have got access to your ConnectionFactory instance, you can inject it into
your components. These components can either be coded against the plain CCI API or
leverage Spring’s support classes for CCI access (e.g. CciTemplate).

When you use a connector in non-managed mode, you can’t use global transactions because
the resource is never enlisted / delisted in the current global transaction of the
current thread. The resource is simply not aware of any global Java EE transactions that
might be running.

5.2.2. ConnectionFactory configuration in Spring

In order to make connections to the EIS, you need to obtain a ConnectionFactory from
the application server if you are in a managed mode, or directly from Spring if you are
in a non-managed mode.

In a managed mode, you access a ConnectionFactory from JNDI; its properties will be
configured in the application server.

<jee:jndi-lookupid="eciConnectionFactory"jndi-name="eis/cicseci"/>

In non-managed mode, you must configure the ConnectionFactory you want to use in the
configuration of Spring as a JavaBean. The LocalConnectionFactoryBean class offers
this setup style, passing in the ManagedConnectionFactory implementation of your
connector, exposing the application-level CCI ConnectionFactory.

You can’t directly instantiate a specific ConnectionFactory. You need to go through
the corresponding implementation of the ManagedConnectionFactory interface for your
connector. This interface is part of the JCA SPI specification.

5.2.3. Configuring CCI connections

JCA CCI allow the developer to configure the connections to the EIS using the
ConnectionSpec implementation of your connector. In order to configure its properties,
you need to wrap the target connection factory with a dedicated adapter,
ConnectionSpecConnectionFactoryAdapter. So, the dedicated ConnectionSpec can be
configured with the property connectionSpec (as an inner bean).

This property is not mandatory because the CCI ConnectionFactory interface defines two
different methods to obtain a CCI connection. Some of the ConnectionSpec properties
can often be configured in the application server (in managed mode) or on the
corresponding local ManagedConnectionFactory implementation.

Spring provides a ConnectionSpecConnectionFactoryAdapter that allows for specifying a
ConnectionSpec instance to use for all operations on a given factory. If the adapter’s
connectionSpec property is specified, the adapter uses the getConnection variant
with the ConnectionSpec argument, otherwise the variant without argument.

5.2.4. Using a single CCI connection

If you want to use a single CCI connection, Spring provides a further
ConnectionFactory adapter to manage this. The SingleConnectionFactory adapter class
will open a single connection lazily and close it when this bean is destroyed at
application shutdown. This class will expose special Connection proxies that behave
accordingly, all sharing the same underlying physical connection.

This ConnectionFactory adapter cannot directly be configured with a ConnectionSpec.
Use an intermediary ConnectionSpecConnectionFactoryAdapter that the
SingleConnectionFactory talks to if you require a single connection for a specific
ConnectionSpec.

5.3. Using Spring’s CCI access support

5.3.1. Record conversion

One of the aims of the JCA CCI support is to provide convenient facilities for
manipulating CCI records. The developer can specify the strategy to create records and
extract datas from records, for use with Spring’s CciTemplate. The following
interfaces will configure the strategy to use input and output records if you don’t want
to work with records directly in your application.

In order to create an input Record, the developer can use a dedicated implementation
of the RecordCreator interface.

As you can see, the createRecord(..) method receives a RecordFactory instance as
parameter, which corresponds to the RecordFactory of the ConnectionFactory used.
This reference can be used to create IndexedRecord or MappedRecord instances. The
following sample shows how to use the RecordCreator interface and indexed/mapped
records.

An output Record can be used to receive data back from the EIS. Hence, a specific
implementation of the RecordExtractor interface can be passed to Spring’s
CciTemplate for extracting data from the output Record.

5.3.2. CciTemplate

The CciTemplate is the central class of the core CCI support package (
org.springframework.jca.cci.core). It simplifies the use of CCI since it handles the
creation and release of resources. This helps to avoid common errors like forgetting to
always close the connection. It cares for the lifecycle of connection and interaction
objects, letting application code focus on generating input records from application
data and extracting application data from output records.

The JCA CCI specification defines two distinct methods to call operations on an EIS. The
CCI Interaction interface provides two execute method signatures:

With the second approach, we need to specify the record creation and record extraction
strategies as arguments. The interfaces used are those describe in the previous section
on record conversion. The corresponding CciTemplate methods are the following:

Unless the outputRecordCreator property is set on the template (see the following
section), every method will call the corresponding execute method of the CCI
Interaction with two parameters: InteractionSpec and input Record, receiving an
output Record as return value.

CciTemplate also provides methods to create IndexRecord and MappedRecord outside a
RecordCreator implementation, through its createIndexRecord(..) and
createMappedRecord(..) methods. This can be used within DAO implementations to create
Record instances to pass into corresponding CciTemplate.execute(..) methods.

5.3.3. DAO support

Spring’s CCI support provides a abstract class for DAOs, supporting injection of a
ConnectionFactory or a CciTemplate instances. The name of the class is
CciDaoSupport: It provides simple setConnectionFactory and setCciTemplate methods.
Internally, this class will create a CciTemplate instance for a passed-in
ConnectionFactory, exposing it to concrete data access implementations in subclasses.

5.3.4. Automatic output record generation

If the connector used only supports the Interaction.execute(..) method with input and
output records as parameters (that is, it requires the desired output record to be
passed in instead of returning an appropriate output record), you can set the
outputRecordCreator property of the CciTemplate to automatically generate an output
record to be filled by the JCA connector when the response is received. This record will
be then returned to the caller of the template.

This property simply holds an implementation of the RecordCreator interface, used for
that purpose. The RecordCreator interface has already been discussed in
Record conversion. The outputRecordCreator property must be directly specified on
the CciTemplate. This could be done in the application code like so:

cciTemplate.setOutputRecordCreator(new EciOutputRecordCreator());

Or (recommended) in the Spring configuration, if the CciTemplate is configured as a
dedicated bean instance:

As the CciTemplate class is thread-safe, it will usually be configured as a shared
instance.

5.3.5. Summary

The following table summarizes the mechanisms of the CciTemplate class and the
corresponding methods called on the CCI Interaction interface:

Table 9. Usage of Interaction execute methods

CciTemplate method signature

CciTemplate outputRecordCreator property

execute method called on the CCI Interaction

Record execute(InteractionSpec, Record)

not set

Record execute(InteractionSpec, Record)

Record execute(InteractionSpec, Record)

set

boolean execute(InteractionSpec, Record, Record)

void execute(InteractionSpec, Record, Record)

not set

void execute(InteractionSpec, Record, Record)

void execute(InteractionSpec, Record, Record)

set

void execute(InteractionSpec, Record, Record)

Record execute(InteractionSpec, RecordCreator)

not set

Record execute(InteractionSpec, Record)

Record execute(InteractionSpec, RecordCreator)

set

void execute(InteractionSpec, Record, Record)

Record execute(InteractionSpec, Record, RecordExtractor)

not set

Record execute(InteractionSpec, Record)

Record execute(InteractionSpec, Record, RecordExtractor)

set

void execute(InteractionSpec, Record, Record)

Record execute(InteractionSpec, RecordCreator, RecordExtractor)

not set

Record execute(InteractionSpec, Record)

Record execute(InteractionSpec, RecordCreator, RecordExtractor)

set

void execute(InteractionSpec, Record, Record)

5.3.6. Using a CCI Connection and Interaction directly

CciTemplate also offers the possibility to work directly with CCI connections and
interactions, in the same manner as JdbcTemplate and JmsTemplate. This is useful
when you want to perform multiple operations on a CCI connection or interaction, for
example.

The interface ConnectionCallback provides a CCI Connection as argument, in order to
perform custom operations on it, plus the CCI ConnectionFactory which the Connection
was created with. The latter can be useful for example to get an associated
RecordFactory instance and create indexed/mapped records, for example.

5.4. Modeling CCI access as operation objects

The org.springframework.jca.cci.object package contains support classes that allow you
to access the EIS in a different style: through reusable operation objects, analogous to
Spring’s JDBC operation objects (see JDBC chapter). This will usually encapsulate the
CCI API: an application-level input object will be passed to the operation object, so it
can construct the input record and then convert the received record data to an
application-level output object and return it.

This approach is internally based on the CciTemplate class and the
RecordCreator / RecordExtractor interfaces, reusing the machinery of Spring’s core
CCI support.

5.4.1. MappingRecordOperation

MappingRecordOperation essentially performs the same work as CciTemplate, but
represents a specific, pre-configured operation as an object. It provides two template
methods to specify how to convert an input object to a input record, and how to convert
an output record to an output object (record mapping):

createInputRecord(..) to specify how to convert an input object to an input Record

extractOutputData(..) to specify how to extract an output object from an output
Record

As you can see, contrary to the CciTemplate class, this execute(..) method does not
have an InteractionSpec as argument. Instead, the InteractionSpec is global to the
operation. The following constructor must be used to instantiate an operation object
with a specific InteractionSpec:

5.4.2. MappingCommAreaOperation

Some connectors use records based on a COMMAREA which represents an array of bytes
containing parameters to send to the EIS and data returned by it. Spring provides a
special operation class for working directly on COMMAREA rather than on records. The
MappingCommAreaOperation class extends the MappingRecordOperation class to provide
such special COMMAREA support. It implicitly uses the CommAreaRecord class as input
and output record type, and provides two new methods to convert an input object into an
input COMMAREA and the output COMMAREA into an output object.

5.4.3. Automatic output record generation

As every MappingRecordOperation subclass is based on CciTemplate internally, the same
way to automatically generate output records as with CciTemplate is available. Every
operation object provides a corresponding setOutputRecordCreator(..) method. For
further information, see Automatic output record generation.

5.4.4. Summary

The operation object approach uses records in the same manner as the CciTemplate class.

Table 10. Usage of Interaction execute methods

MappingRecordOperation method signature

MappingRecordOperation outputRecordCreator property

execute method called on the CCI Interaction

Object execute(Object)

not set

Record execute(InteractionSpec, Record)

Object execute(Object)

set

boolean execute(InteractionSpec, Record, Record)

5.4.5. Example for MappingRecordOperation usage

In this section, the usage of the MappingRecordOperation will be shown to access a
database with the Blackbox CCI connector.

The original version of this connector is provided by the Java EE SDK (version 1.3),
available from Oracle.

Firstly, some initializations on the CCI InteractionSpec must be done to specify which
SQL request to execute. In this sample, we directly define the way to convert the
parameters of the request to a CCI record and the way to convert the CCI result record
to an instance of the Person class.

5.5. Transactions

JCA specifies several levels of transaction support for resource adapters. The kind of
transactions that your resource adapter supports is specified in its ra.xml file.
There are essentially three options: none (for example with CICS EPI connector), local
transactions (for example with a CICS ECI connector), global transactions (for example
with an IMS connector).

For global transactions, you can use Spring’s generic transaction infrastructure to
demarcate transactions, with JtaTransactionManager as backend (delegating to the Java
EE server’s distributed transaction coordinator underneath).

For local transactions on a single CCI ConnectionFactory, Spring provides a specific
transaction management strategy for CCI, analogous to the DataSourceTransactionManager
for JDBC. The CCI API defines a local transaction object and corresponding local
transaction demarcation methods. Spring’s CciLocalTransactionManager executes such
local CCI transactions, fully compliant with Spring’s generic
PlatformTransactionManager abstraction.

Both transaction strategies can be used with any of Spring’s transaction demarcation
facilities, be it declarative or programmatic. This is a consequence of Spring’s generic
PlatformTransactionManager abstraction, which decouples transaction demarcation from
the actual execution strategy. Simply switch between JtaTransactionManager and
CciLocalTransactionManager as needed, keeping your transaction demarcation as-is.

6. Email

6.1. Introduction

This library is freely available on the web — for example, in Maven Central as
com.sun.mail:javax.mail.

The Spring Framework provides a helpful utility library for sending email that shields
the user from the specifics of the underlying mailing system and is responsible for low
level resource handling on behalf of the client.

The org.springframework.mail package is the root level package for the Spring
Framework’s email support. The central interface for sending emails is the MailSender
interface; a simple value object encapsulating the properties of a simple mail such as
from and to (plus many others) is the SimpleMailMessage class. This package
also contains a hierarchy of checked exceptions which provide a higher level of
abstraction over the lower level mail system exceptions with the root exception being
MailException. Please refer to the javadocs for more information on the rich mail
exception hierarchy.

The org.springframework.mail.javamail.JavaMailSender interface adds specialized
JavaMail features such as MIME message support to the MailSender interface (from
which it inherits). JavaMailSender also provides a callback interface for preparing
a 'MimeMessage', called org.springframework.mail.javamail.MimeMessagePreparator.

6.2. Usage

Let’s assume there is a business interface called OrderManager:

publicinterfaceOrderManager {
void placeOrder(Order order);
}

Let us also assume that there is a requirement stating that an email message with an
order number needs to be generated and sent to a customer placing the relevant order.

<beanid="mailSender"class="org.springframework.mail.javamail.JavaMailSenderImpl"><propertyname="host"value="mail.mycompany.com"/></bean><!-- this is a template message that we can pre-load with default state --><beanid="templateMessage"class="org.springframework.mail.SimpleMailMessage"><propertyname="from"value="[email protected]"/><propertyname="subject"value="Your order"/></bean><beanid="orderManager"class="com.mycompany.businessapp.support.SimpleOrderManager"><propertyname="mailSender"ref="mailSender"/><propertyname="templateMessage"ref="templateMessage"/></bean>

6.2.2. Using the JavaMailSender and the MimeMessagePreparator

Here is another implementation of OrderManager using the MimeMessagePreparator
callback interface. Please note in this case that the mailSender property is of type
JavaMailSender so that we are able to use the JavaMail MimeMessage class:

The mail code is a crosscutting concern and could well be a candidate for refactoring
into a custom Spring AOP aspect, which then could be executed at appropriate
joinpoints on the OrderManager target.

The Spring Framework’s mail support ships with the standard JavaMail implementation.
Please refer to the relevant javadocs for more information.

6.3. Using the JavaMail MimeMessageHelper

A class that comes in pretty handy when dealing with JavaMail messages is the
org.springframework.mail.javamail.MimeMessageHelper class, which shields you from
having to use the verbose JavaMail API. Using the MimeMessageHelper it is pretty easy
to create a MimeMessage:

// of course you would use DI in any real-world cases
JavaMailSenderImpl sender = new JavaMailSenderImpl();
sender.setHost("mail.host.com");
MimeMessage message = sender.createMimeMessage();
MimeMessageHelper helper = new MimeMessageHelper(message);
helper.setTo("[email protected]");
helper.setText("Thank you for ordering!");
sender.send(message);

6.3.1. Sending attachments and inline resources

Multipart email messages allow for both attachments and inline resources. Examples of
inline resources would be images or a stylesheet you want to use in your message, but
that you don’t want displayed as an attachment.

Attachments

The following example shows you how to use the MimeMessageHelper to send an email
along with a single JPEG image attachment.

Inline resources are added to the MimeMessage using the specified Content-ID
(identifier1234 in the above example). The order in which you are adding the text
and the resource are very important. Be sure to first add the text and after
that the resources. If you are doing it the other way around, it won’t work!

6.3.2. Creating email content using a templating library

The code in the previous examples explicitly created the content of the email message,
using methods calls such as message.setText(..). This is fine for simple cases, and it
is okay in the context of the aforementioned examples, where the intent was to show you
the very basics of the API.

In your typical enterprise application though, you are not going to create the content
of your emails using the above approach for a number of reasons.

Typically the approach taken to address these issues is to use a template library such
as FreeMarker to define the display structure of email content. This leaves your code
tasked only with creating the data that is to be rendered in the email template and
sending the email. It is definitely a best practice for when the content of your emails
becomes even moderately complex, and with the Spring Framework’s support classes for
FreeMarker becomes quite easy to do.

7. Task Execution and Scheduling

7.1. Introduction

The Spring Framework provides abstractions for asynchronous execution and scheduling of
tasks with the TaskExecutor and TaskScheduler interfaces, respectively. Spring also
features implementations of those interfaces that support thread pools or delegation to
CommonJ within an application server environment. Ultimately the use of these
implementations behind the common interfaces abstracts away the differences between Java
SE 5, Java SE 6 and Java EE environments.

Spring also features integration classes for supporting scheduling with the Timer,
part of the JDK since 1.3, and the Quartz Scheduler ( http://quartz-scheduler.org).
Both of those schedulers are set up using a FactoryBean with optional references to
Timer or Trigger instances, respectively. Furthermore, a convenience class for both
the Quartz Scheduler and the Timer is available that allows you to invoke a method of
an existing target object (analogous to the normal MethodInvokingFactoryBean
operation).

7.2. The Spring TaskExecutor abstraction

Executors are the JDK name for the concept of thread pools. The "executor" naming is
due to the fact that there is no guarantee that the underlying implementation is
actually a pool; an executor may be single-threaded or even synchronous. Spring’s
abstraction hides implementation details between Java SE and Java EE environments.

Spring’s TaskExecutor interface is identical to the java.util.concurrent.Executor
interface. In fact, originally, its primary reason for existence was to abstract away
the need for Java 5 when using thread pools. The interface has a single method
execute(Runnable task) that accepts a task for execution based on the semantics
and configuration of the thread pool.

The TaskExecutor was originally created to give other Spring components an abstraction
for thread pooling where needed. Components such as the ApplicationEventMulticaster,
JMS’s AbstractMessageListenerContainer, and Quartz integration all use the
TaskExecutor abstraction to pool threads. However, if your beans need thread pooling
behavior, it is possible to use this abstraction for your own needs.

7.2.1. TaskExecutor types

There are a number of pre-built implementations of TaskExecutor included with the
Spring distribution. In all likelihood, you shouldn’t ever need to implement your own.

SimpleAsyncTaskExecutor
This implementation does not reuse any threads, rather it starts up a new thread
for each invocation. However, it does support a concurrency limit which will block
any invocations that are over the limit until a slot has been freed up. If you
are looking for true pooling, see the discussions of SimpleThreadPoolTaskExecutor
and ThreadPoolTaskExecutor below.

SyncTaskExecutor
This implementation doesn’t execute invocations asynchronously. Instead, each
invocation takes place in the calling thread. It is primarily used in situations
where multi-threading isn’t necessary such as simple test cases.

ConcurrentTaskExecutor
This implementation is an adapter for a java.util.concurrent.Executor object.
There is an alternative, ThreadPoolTaskExecutor, that exposes the Executor
configuration parameters as bean properties. It is rare to need to use the
ConcurrentTaskExecutor, but if the ThreadPoolTaskExecutor isn’t flexible
enough for your needs, the ConcurrentTaskExecutor is an alternative.

SimpleThreadPoolTaskExecutor
This implementation is actually a subclass of Quartz’s SimpleThreadPool which
listens to Spring’s lifecycle callbacks. This is typically used when you have a
thread pool that may need to be shared by both Quartz and non-Quartz components.

ThreadPoolTaskExecutor
This implementation is the most commonly used one. It exposes bean properties for
configuring a java.util.concurrent.ThreadPoolExecutor and wraps it in a TaskExecutor.
If you need to adapt to a different kind of java.util.concurrent.Executor, it is
recommended that you use a ConcurrentTaskExecutor instead.

WorkManagerTaskExecutor

CommonJ is a set of specifications jointly developed between BEA and IBM. These
specifications are not Java EE standards, but are standard across BEA’s and IBM’s
Application Server implementations.

This implementation uses the CommonJ WorkManager as its backing implementation and is
the central convenience class for setting up a CommonJ WorkManager reference in a Spring
context. Similar to the SimpleThreadPoolTaskExecutor, this class implements the
WorkManager interface and therefore can be used directly as a WorkManager as well.

7.2.2. Using a TaskExecutor

Spring’s TaskExecutor implementations are used as simple JavaBeans. In the example
below, we define a bean that uses the ThreadPoolTaskExecutor to asynchronously print
out a set of messages.

As you can see, rather than retrieving a thread from the pool and executing yourself,
you add your Runnable to the queue and the TaskExecutor uses its internal rules to
decide when the task gets executed.

To configure the rules that the TaskExecutor will use, simple bean properties have
been exposed.

The simplest method is the one named 'schedule' that takes a Runnable and Date only.
That will cause the task to run once after the specified time. All of the other methods
are capable of scheduling tasks to run repeatedly. The fixed-rate and fixed-delay
methods are for simple, periodic execution, but the method that accepts a Trigger is
much more flexible.

7.3.1. Trigger interface

The Trigger interface is essentially inspired by JSR-236, which, as of Spring 3.0, has
not yet been officially implemented. The basic idea of the Trigger is that execution
times may be determined based on past execution outcomes or even arbitrary conditions.
If these determinations do take into account the outcome of the preceding execution,
that information is available within a TriggerContext. The Trigger interface itself
is quite simple:

As you can see, the TriggerContext is the most important part. It encapsulates all of
the relevant data, and is open for extension in the future if necessary. The
TriggerContext is an interface (a SimpleTriggerContext implementation is used by
default). Here you can see what methods are available for Trigger implementations.

7.3.2. Trigger implementations

Spring provides two implementations of the Trigger interface. The most interesting one
is the CronTrigger. It enables the scheduling of tasks based on cron expressions. For
example, the following task is being scheduled to run 15 minutes past each hour but only
during the 9-to-5 "business hours" on weekdays.

scheduler.schedule(task, new CronTrigger("0 15 9-17 * * MON-FRI"));

The other out-of-the-box implementation is a PeriodicTrigger that accepts a fixed
period, an optional initial delay value, and a boolean to indicate whether the period
should be interpreted as a fixed-rate or a fixed-delay. Since the TaskScheduler
interface already defines methods for scheduling tasks at a fixed-rate or with a
fixed-delay, those methods should be used directly whenever possible. The value of the
PeriodicTrigger implementation is that it can be used within components that rely on
the Trigger abstraction. For example, it may be convenient to allow periodic triggers,
cron-based triggers, and even custom trigger implementations to be used interchangeably.
Such a component could take advantage of dependency injection so that such Triggers
could be configured externally and therefore easily modified or extended.

7.3.3. TaskScheduler implementations

As with Spring’s TaskExecutor abstraction, the primary benefit of the TaskScheduler
is that code relying on scheduling behavior need not be coupled to a particular
scheduler implementation. The flexibility this provides is particularly relevant when
running within Application Server environments where threads should not be created
directly by the application itself. For such cases, Spring provides a
TimerManagerTaskScheduler that delegates to a CommonJ TimerManager instance, typically
configured with a JNDI-lookup.

A simpler alternative, the ThreadPoolTaskScheduler, can be used whenever external
thread management is not a requirement. Internally, it delegates to a
ScheduledExecutorService instance. ThreadPoolTaskScheduler actually implements
Spring’s TaskExecutor interface as well, so that a single instance can be used for
asynchronous execution as soon as possible as well as scheduled, and potentially
recurring, executions.

7.4. Annotation Support for Scheduling and Asynchronous Execution

Spring provides annotation support for both task scheduling and asynchronous method
execution.

7.4.1. Enable scheduling annotations

To enable support for @Scheduled and @Async annotations add @EnableScheduling and
@EnableAsync to one of your @Configuration classes:

@Configuration@EnableAsync@EnableSchedulingpublicclassAppConfig {
}

You are free to pick and choose the relevant annotations for your application. For
example, if you only need support for @Scheduled, simply omit @EnableAsync. For more
fine-grained control you can additionally implement the SchedulingConfigurer and/or
AsyncConfigurer interfaces. See the javadocs for full details.

If you prefer XML configuration use the <task:annotation-driven> element.

Notice with the above XML that an executor reference is provided for handling those
tasks that correspond to methods with the @Async annotation, and the scheduler
reference is provided for managing those methods annotated with @Scheduled.

The default advice mode for processing @Async annotations is "proxy" which allows
for interception of calls through the proxy only; local calls within the same class
cannot get intercepted that way. For a more advanced mode of interception, consider
switching to "aspectj" mode in combination with compile-time or load-time weaving.

7.4.2. The @Scheduled annotation

The @Scheduled annotation can be added to a method along with trigger metadata. For
example, the following method would be invoked every 5 seconds with a fixed delay,
meaning that the period will be measured from the completion time of each preceding
invocation.

If a fixed rate execution is desired, simply change the property name specified within
the annotation. The following would be executed every 5 seconds measured between the
successive start times of each invocation.

You can additionally use the zone attribute to specify the time zone in which the cron
expression will be resolved.

Notice that the methods to be scheduled must have void returns and must not expect any
arguments. If the method needs to interact with other objects from the Application
Context, then those would typically have been provided through dependency injection.

As of Spring Framework 4.3, @Scheduled methods are supported on beans of any scope.

Make sure that you are not initializing multiple instances of the same @Scheduled
annotation class at runtime, unless you do want to schedule callbacks to each such
instance. Related to this, make sure that you do not use @Configurable on bean
classes which are annotated with @Scheduled and registered as regular Spring beans
with the container: You would get double initialization otherwise, once through the
container and once through the @Configurable aspect, with the consequence of each
@Scheduled method being invoked twice.

7.4.3. The @Async annotation

The @Async annotation can be provided on a method so that invocation of that method
will occur asynchronously. In other words, the caller will return immediately upon
invocation and the actual execution of the method will occur in a task that has been
submitted to a Spring TaskExecutor. In the simplest case, the annotation may be
applied to a void-returning method.

@Asyncvoid doSomething() {
// this will be executed asynchronously
}

Unlike the methods annotated with the @Scheduled annotation, these methods can expect
arguments, because they will be invoked in the "normal" way by callers at runtime rather
than from a scheduled task being managed by the container. For example, the following is
a legitimate application of the @Async annotation.

Even methods that return a value can be invoked asynchronously. However, such methods
are required to have a Future typed return value. This still provides the benefit of
asynchronous execution so that the caller can perform other tasks prior to calling
get() on that Future.

@Async methods may not only declare a regular java.util.concurrent.Future return type
but also Spring’s org.springframework.util.concurrent.ListenableFuture or, as of Spring
4.2, JDK 8’s java.util.concurrent.CompletableFuture: for richer interaction with the
asynchronous task and for immediate composition with further processing steps.

@Async can not be used in conjunction with lifecycle callbacks such as
@PostConstruct. To asynchronously initialize Spring beans you currently have to use
a separate initializing Spring bean that invokes the @Async annotated method on the
target then.

There is no direct XML equivalent for @Async since such methods should be designed
for asynchronous execution in the first place, not externally re-declared to be async.
However, you may manually set up Spring’s AsyncExecutionInterceptor with Spring AOP,
in combination with a custom pointcut.

7.4.4. Executor qualification with @Async

By default when specifying @Async on a method, the executor that will be used is the
one supplied to the 'annotation-driven' element as described above. However, the value
attribute of the @Async annotation can be used when needing to indicate that an
executor other than the default should be used when executing a given method.

In this case, "otherExecutor" may be the name of any Executor bean in the Spring
container, or may be the name of a qualifier associated with any Executor, e.g. as
specified with the <qualifier> element or Spring’s @Qualifier annotation.

7.4.5. Exception management with @Async

When an @Async method has a Future typed return value, it is easy to manage
an exception that was thrown during the method execution as this exception will
be thrown when calling get on the Future result. With a void return type
however, the exception is uncaught and cannot be transmitted. For those cases, an
AsyncUncaughtExceptionHandler can be provided to handle such exceptions.

By default, the exception is simply logged. A custom AsyncUncaughtExceptionHandler can
be defined viaAsyncConfigurer or the task:annotation-driven XML element.

7.5. The task namespace

Beginning with Spring 3.0, there is an XML namespace for configuring TaskExecutor and
TaskScheduler instances. It also provides a convenient way to configure tasks to be
scheduled with a trigger.

7.5.1. The 'scheduler' element

The following element will create a ThreadPoolTaskScheduler instance with the
specified thread pool size.

<task:schedulerid="scheduler"pool-size="10"/>

The value provided for the 'id' attribute will be used as the prefix for thread names
within the pool. The 'scheduler' element is relatively straightforward. If you do not
provide a 'pool-size' attribute, the default thread pool will only have a single thread.
There are no other configuration options for the scheduler.

7.5.2. The 'executor' element

The following will create a ThreadPoolTaskExecutor instance:

<task:executorid="executor"pool-size="10"/>

As with the scheduler above, the value provided for the 'id' attribute will be used as
the prefix for thread names within the pool. As far as the pool size is concerned, the
'executor' element supports more configuration options than the 'scheduler' element. For
one thing, the thread pool for a ThreadPoolTaskExecutor is itself more configurable.
Rather than just a single size, an executor’s thread pool may have different values for
the core and the max size. If a single value is provided then the executor will
have a fixed-size thread pool (the core and max sizes are the same). However, the
'executor' element’s 'pool-size' attribute also accepts a range in the form of "min-max".

As you can see from that configuration, a 'queue-capacity' value has also been provided.
The configuration of the thread pool should also be considered in light of the
executor’s queue capacity. For the full description of the relationship between pool
size and queue capacity, consult the documentation for
ThreadPoolExecutor.
The main idea is that when a task is submitted, the executor will first try to use a
free thread if the number of active threads is currently less than the core size. If the
core size has been reached, then the task will be added to the queue as long as its
capacity has not yet been reached. Only then, if the queue’s capacity has been
reached, will the executor create a new thread beyond the core size. If the max size has
also been reached, then the executor will reject the task.

By default, the queue is unbounded, but this is rarely the desired configuration,
because it can lead to OutOfMemoryErrors if enough tasks are added to that queue while
all pool threads are busy. Furthermore, if the queue is unbounded, then the max size has
no effect at all. Since the executor will always try the queue before creating a new
thread beyond the core size, a queue must have a finite capacity for the thread pool to
grow beyond the core size (this is why a fixed size pool is the only sensible case
when using an unbounded queue).

In a moment, we will review the effects of the keep-alive setting which adds yet another
factor to consider when providing a pool size configuration. First, let’s consider the
case, as mentioned above, when a task is rejected. By default, when a task is rejected,
a thread pool executor will throw a TaskRejectedException. However, the rejection
policy is actually configurable. The exception is thrown when using the default
rejection policy which is the AbortPolicy implementation. For applications where some
tasks can be skipped under heavy load, either the DiscardPolicy or
DiscardOldestPolicy may be configured instead. Another option that works well for
applications that need to throttle the submitted tasks under heavy load is the
CallerRunsPolicy. Instead of throwing an exception or discarding tasks, that policy
will simply force the thread that is calling the submit method to run the task itself.
The idea is that such a caller will be busy while running that task and not able to
submit other tasks immediately. Therefore it provides a simple way to throttle the
incoming load while maintaining the limits of the thread pool and queue. Typically this
allows the executor to "catch up" on the tasks it is handling and thereby frees up some
capacity on the queue, in the pool, or both. Any of these options can be chosen from an
enumeration of values available for the 'rejection-policy' attribute on the 'executor'
element.

Finally, the keep-alive setting determines the time limit (in seconds) for which threads
may remain idle before being terminated. If there are more than the core number of threads
currently in the pool, after waiting this amount of time without processing a task, excess
threads will get terminated. A time value of zero will cause excess threads to terminate
immediately after executing a task without remaining follow-up work in the task queue.

7.5.3. The 'scheduled-tasks' element

The most powerful feature of Spring’s task namespace is the support for configuring
tasks to be scheduled within a Spring Application Context. This follows an approach
similar to other "method-invokers" in Spring, such as that provided by the JMS namespace
for configuring Message-driven POJOs. Basically a "ref" attribute can point to any
Spring-managed object, and the "method" attribute provides the name of a method to be
invoked on that object. Here is a simple example.

As you can see, the scheduler is referenced by the outer element, and each individual
task includes the configuration of its trigger metadata. In the preceding example, that
metadata defines a periodic trigger with a fixed delay indicating the number of
milliseconds to wait after each task execution has completed. Another option is
'fixed-rate', indicating how often the method should be executed regardless of how long
any previous execution takes. Additionally, for both fixed-delay and fixed-rate tasks an
'initial-delay' parameter may be specified indicating the number of milliseconds to wait
before the first execution of the method. For more control, a "cron" attribute may be
provided instead. Here is an example demonstrating these other options.

7.6. Using the Quartz Scheduler

Quartz uses Trigger, Job and JobDetail objects to realize scheduling of all kinds
of jobs. For the basic concepts behind Quartz, have a look at
http://quartz-scheduler.org. For convenience purposes, Spring offers a couple of
classes that simplify the usage of Quartz within Spring-based applications.

7.6.1. Using the JobDetailFactoryBean

Quartz JobDetail objects contain all information needed to run a job. Spring provides a
JobDetailFactoryBean which provides bean-style properties for XML configuration purposes.
Let’s have a look at an example:

The job detail configuration has all information it needs to run the job (ExampleJob).
The timeout is specified in the job data map. The job data map is available through the
JobExecutionContext (passed to you at execution time), but the JobDetail also gets
its properties from the job data mapped to properties of the job instance. So in this
case, if the ExampleJob contains a bean property named timeout, the JobDetail
will have it applied automatically:

All additional properties from the job data map are of course available to you as well.

Using the name and group properties, you can modify the name and the group
of the job, respectively. By default, the name of the job matches the bean name
of the JobDetailFactoryBean (in the example above, this is exampleJob).

7.6.2. Using the MethodInvokingJobDetailFactoryBean

Often you just need to invoke a method on a specific object. Using the
MethodInvokingJobDetailFactoryBean you can do exactly this:

Using the MethodInvokingJobDetailFactoryBean, you don’t need to create one-line jobs
that just invoke a method, and you only need to create the actual business object and
wire up the detail object.

By default, Quartz Jobs are stateless, resulting in the possibility of jobs interfering
with each other. If you specify two triggers for the same JobDetail, it might be
possible that before the first job has finished, the second one will start. If
JobDetail classes implement the Stateful interface, this won’t happen. The second
job will not start before the first one has finished. To make jobs resulting from the
MethodInvokingJobDetailFactoryBean non-concurrent, set the concurrent flag to
false.

7.6.3. Wiring up jobs using triggers and the SchedulerFactoryBean

We’ve created job details and jobs. We’ve also reviewed the convenience bean that allows
you to invoke a method on a specific object. Of course, we still need to schedule the
jobs themselves. This is done using triggers and a SchedulerFactoryBean. Several
triggers are available within Quartz and Spring offers two Quartz FactoryBean
implementations with convenient defaults: CronTriggerFactoryBean and
SimpleTriggerFactoryBean.

Triggers need to be scheduled. Spring offers a SchedulerFactoryBean that exposes
triggers to be set as properties. SchedulerFactoryBean schedules the actual jobs with
those triggers.

More properties are available for the SchedulerFactoryBean for you to set, such as the
calendars used by the job details, properties to customize Quartz with, etc. Have a look
at the
SchedulerFactoryBean
javadocs for more information.

8. Cache Abstraction

8.1. Introduction

Since version 3.1, Spring Framework provides support for transparently adding caching
into an existing Spring application. Similar to the transaction
support, the caching abstraction allows consistent use of various caching solutions with minimal
impact on the code.

As from Spring 4.1, the cache abstraction has been significantly improved with the
support of JSR-107 annotations and more customization options.

8.2. Understanding the cache abstraction

Cache vs Buffer

The terms "buffer" and "cache" tend to be used interchangeably; note however they
represent different things. A buffer is used traditionally as an intermediate temporary
store for data between a fast and a slow entity. As one party would have to wait for
the other affecting performance, the buffer alleviates this by allowing entire blocks of
data to move at once rather then in small chunks. The data is written and read only once
from the buffer. Furthermore, the buffers are visible to at least one party which is
aware of it.

A cache on the other hand by definition is hidden and neither party is aware that
caching occurs.It as well improves performance but does that by allowing the same data
to be read multiple times in a fast fashion.

A further explanation of the differences between two can be found
here.

At its core, the abstraction applies caching to Java methods, reducing thus the number
of executions based on the information available in the cache. That is, each time a
targeted method is invoked, the abstraction will apply a caching behavior checking
whether the method has been already executed for the given arguments. If it has, then
the cached result is returned without having to execute the actual method; if it has
not, then method is executed, the result cached and returned to the user so that, the
next time the method is invoked, the cached result is returned. This way, expensive
methods (whether CPU or IO bound) can be executed only once for a given set of
parameters and the result reused without having to actually execute the method again.
The caching logic is applied transparently without any interference to the invoker.

Obviously this approach works only for methods that are guaranteed to return the same
output (result) for a given input (or arguments) no matter how many times it is being
executed.

Other cache-related operations are provided by the abstraction such as the ability
to update the content of the cache or remove one of all entries. These are useful if
the cache deals with data that can change during the course of the application.

Just like other services in the Spring Framework, the caching service is an
abstraction (not a cache implementation) and requires the use of an actual storage to
store the cache data - that is, the abstraction frees the developer from having to write
the caching logic but does not provide the actual stores. This abstraction is
materialized by the org.springframework.cache.Cache and
org.springframework.cache.CacheManager interfaces.

The caching abstraction has no special handling of multi-threaded and multi-process
environments as such features are handled by the cache implementation. .

If you have a multi-process environment (i.e. an application deployed on several nodes),
you will need to configure your cache provider accordingly. Depending on your use cases,
a copy of the same data on several nodes may be enough but if you change the data during
the course of the application, you may need to enable other propagation mechanisms.

Caching a particular item is a direct equivalent of the typical get-if-not-found-then-
proceed-and-put-eventually code blocks found with programmatic cache interaction: no locks
are applied and several threads may try to load the same item concurrently. The same applies
to eviction: if several threads are trying to update or evict data concurrently, you may
use stale data. Certain cache providers offer advanced features in that area, refer to
the documentation of the cache provider that you are using for more details.

To use the cache abstraction, the developer needs to take care of two aspects:

caching declaration - identify the methods that need to be cached and their policy

cache configuration - the backing cache where the data is stored and read from

8.3. Declarative annotation-based caching

For caching declaration, the abstraction provides a set of Java annotations:

@Cacheable triggers cache population

@CacheEvict triggers cache eviction

@CachePut updates the cache without interfering with the method execution

@Caching regroups multiple cache operations to be applied on a method

@CacheConfig shares some common cache-related settings at class-level

Let us take a closer look at each annotation:

8.3.1. @Cacheable annotation

As the name implies, @Cacheable is used to demarcate methods that are cacheable - that
is, methods for whom the result is stored into the cache so on subsequent invocations
(with the same arguments), the value in the cache is returned without having to actually
execute the method. In its simplest form, the annotation declaration requires the name
of the cache associated with the annotated method:

@Cacheable("books")
publicBook findBook(ISBN isbn) {...}

In the snippet above, the method findBook is associated with the cache named books.
Each time the method is called, the cache is checked to see whether the invocation has
been already executed and does not have to be repeated. While in most cases, only one
cache is declared, the annotation allows multiple names to be specified so that more
than one cache are being used. In this case, each of the caches will be checked before
executing the method - if at least one cache is hit, then the associated value will be
returned:

All the other caches that do not contain the value will be updated as well even though
the cached method was not actually executed.

@Cacheable({"books", "isbns"})
publicBook findBook(ISBN isbn) {...}

Default Key Generation

Since caches are essentially key-value stores, each invocation of a cached method needs
to be translated into a suitable key for cache access. Out of the box, the caching
abstraction uses a simple KeyGenerator based on the following algorithm:

If no params are given, return SimpleKey.EMPTY.

If only one param is given, return that instance.

If more the one param is given, return a SimpleKey containing all parameters.

This approach works well for most use-cases; As long as parameters have natural keys
and implement valid hashCode() and equals() methods. If that is not the case then the
strategy needs to be changed.

To provide a different default key generator, one needs to implement the
org.springframework.cache.interceptor.KeyGenerator interface.

The default key generation strategy changed with the release of Spring 4.0. Earlier
versions of Spring used a key generation strategy that, for multiple key parameters,
only considered the hashCode() of parameters and not equals(); this could cause
unexpected key collisions (see SPR-10237
for background). The new 'SimpleKeyGenerator' uses a compound key for such scenarios.

If you want to keep using the previous key strategy, you can configure the deprecated
org.springframework.cache.interceptor.DefaultKeyGenerator class or create a custom
hash-based 'KeyGenerator' implementation.

Custom Key Generation Declaration

Since caching is generic, it is quite likely the target methods have various signatures
that cannot be simply mapped on top of the cache structure. This tends to become obvious
when the target method has multiple arguments out of which only some are suitable for
caching (while the rest are used only by the method logic). For example:

At first glance, while the two boolean arguments influence the way the book is found,
they are no use for the cache. Further more what if only one of the two is important
while the other is not?

For such cases, the @Cacheable annotation allows the user to specify how the key is
generated through its key attribute. The developer can use SpEL to
pick the arguments of interest (or their nested properties), perform operations or even
invoke arbitrary methods without having to write any code or implement any interface.
This is the recommended approach over the
default generator since methods tend to be
quite different in signatures as the code base grows; while the default strategy might
work for some methods, it rarely does for all methods.

Below are some examples of various SpEL declarations - if you are not familiar with it,
do yourself a favor and read Spring Expression Language:

The snippets above show how easy it is to select a certain argument, one of its
properties or even an arbitrary (static) method.

If the algorithm responsible to generate the key is too specific or if it needs
to be shared, you may define a custom keyGenerator on the operation. To do
this, specify the name of the KeyGenerator bean implementation to use:

It is also possible to replace the CacheResolver entirely in a similar
fashion as for key generation. The
resolution is requested for every cache operation, giving a chance to
the implementation to actually resolve the cache(s) to use based on
runtime arguments:

Since Spring 4.1, the value attribute of the cache annotations are no longer
mandatory since this particular information can be provided by the CacheResolver
regardless of the content of the annotation.

Similarly to key and keyGenerator, the cacheManager and cacheResolver
parameters are mutually exclusive and an operation specifying both will
result in an exception as a custom CacheManager will be ignored by the
CacheResolver implementation. This is probably not what you expect.

Synchronized caching

In a multi-threaded environment, certain operations might be concurrently invoked for
the same argument (typically on startup). By default, the cache abstraction does not
lock anything and the same value may be computed several times, defeating the purpose
of caching.

For those particular cases, the sync attribute can be used to instruct the underlying
cache provider to lock the cache entry while the value is being computed. As a result,
only one thread will be busy computing the value while the others are blocked until the
entry is updated in the cache.

This is an optional feature and your favorite cache library may not support it. All
CacheManager implementations provided by the core framework support it. Check the
documentation of your cache provider for more details.

Conditional caching

Sometimes, a method might not be suitable for caching all the time (for example, it
might depend on the given arguments). The cache annotations support such functionality
through the condition parameter which takes a SpEL expression that is evaluated to
either true or false. If true, the method is cached - if not, it behaves as if the
method is not cached, that is executed every time no matter what values are in the cache
or what arguments are used. A quick example - the following method will be cached only
if the argument name has a length shorter than 32:

In addition the condition parameter, the unless parameter can be used to veto the
adding of a value to the cache. Unlike condition, unless expressions are evaluated
after the method has been called. Expanding on the previous example - perhaps we
only want to cache paperback books:

The cache abstraction supports java.util.Optional, using its content as cached value
only if it present. #result always refers to the business entity and never on a
supported wrapper so the previous example can be rewritten as follows:

Note that result still refers to Book and not Optional. As it might be null, we
should use the safe navigation operator.

Available caching SpEL evaluation context

Each SpEL expression evaluates again a dedicated
context. In addition to the build in parameters, the
framework provides dedicated caching related metadata such as the argument names. The
next table lists the items made available to the context so one can use them for key and
conditional computations:

Table 11. Cache SpEL available metadata

Name

Location

Description

Example

methodName

root object

The name of the method being invoked

#root.methodName

method

root object

The method being invoked

#root.method.name

target

root object

The target object being invoked

#root.target

targetClass

root object

The class of the target being invoked

#root.targetClass

args

root object

The arguments (as array) used for invoking the target

#root.args[0]

caches

root object

Collection of caches against which the current method is executed

#root.caches[0].name

argument name

evaluation context

Name of any of the method arguments. If for some reason the names are not available
(e.g. no debug information), the argument names are also available under the #a<#arg>
where #arg stands for the argument index (starting from 0).

#iban or #a0 (one can also use #p0 or #p<#arg> notation as an alias).

result

evaluation context

The result of the method call (the value to be cached). Only available in unless
expressions, cache put expressions (to compute the key), or cache evict
expressions (when beforeInvocation is false). For supported wrappers such as
Optional, #result refers to the actual object, not the wrapper.

#result

8.3.2. @CachePut annotation

For cases where the cache needs to be updated without interfering with the method
execution, one can use the @CachePut annotation. That is, the method will always be
executed and its result placed into the cache (according to the @CachePut options). It
supports the same options as @Cacheable and should be used for cache population rather
than method flow optimization:

Note that using @CachePut and @Cacheable annotations on the same method is generally
strongly discouraged because they have different behaviors. While the latter causes the
method execution to be skipped by using the cache, the former forces the execution in
order to execute a cache update. This leads to unexpected behavior and with the exception of
specific corner-cases (such as annotations having conditions that exclude them from each
other), such declaration should be avoided. Note also that such condition should not rely
on the result object (i.e. the #result variable) as these are validated upfront to confirm
the exclusion.

8.3.3. @CacheEvict annotation

The cache abstraction allows not just population of a cache store but also eviction.
This process is useful for removing stale or unused data from the cache. Opposed to
@Cacheable, annotation @CacheEvict demarcates methods that perform cache
eviction, that is methods that act as triggers for removing data from the cache.
Just like its sibling, @CacheEvict requires specifying one (or multiple) caches
that are affected by the action, allows a custom cache and key resolution or a
condition to be specified but in addition, features an extra parameter
allEntries which indicates whether a cache-wide eviction needs to be performed
rather then just an entry one (based on the key):

This option comes in handy when an entire cache region needs to be cleared out - rather
then evicting each entry (which would take a long time since it is inefficient), all the
entries are removed in one operation as shown above. Note that the framework will ignore
any key specified in this scenario as it does not apply (the entire cache is evicted not
just one entry).

One can also indicate whether the eviction should occur after (the default) or before
the method executes through the beforeInvocation attribute. The former provides the
same semantics as the rest of the annotations - once the method completes successfully,
an action (in this case eviction) on the cache is executed. If the method does not
execute (as it might be cached) or an exception is thrown, the eviction does not occur.
The latter ( beforeInvocation=true) causes the eviction to occur always, before the
method is invoked - this is useful in cases where the eviction does not need to be tied
to the method outcome.

It is important to note that void methods can be used with @CacheEvict - as the
methods act as triggers, the return values are ignored (as they don’t interact with the
cache) - this is not the case with @Cacheable which adds/updates data into the cache
and thus requires a result.

8.3.4. @Caching annotation

There are cases when multiple annotations of the same type, such as @CacheEvict or
@CachePut need to be specified, for example because the condition or the key
expression is different between different caches. @Caching allows multiple nested
@Cacheable, @CachePut and @CacheEvict to be used on the same method:

8.3.5. @CacheConfig annotation

So far we have seen that caching operations offered many customization options and
these can be set on an operation basis. However, some of the customization options
can be tedious to configure if they apply to all operations of the class. For
instance, specifying the name of the cache to use for every cache operation of the
class could be replaced by a single class-level definition. This is where @CacheConfig
comes into play.

@CacheConfig is a class-level annotation that allows to share the cache names, the custom
KeyGenerator, the custom CacheManager and finally the custom CacheResolver. Placing
this annotation on the class does not turn on any caching operation.

An operation-level customization will always override a customization set on @CacheConfig. This
gives therefore three levels of customizations per cache operation:

Globally configured, available for CacheManager, KeyGenerator

At class level, using @CacheConfig

At the operation level

8.3.6. Enable caching annotations

It is important to note that even though declaring the cache annotations does not
automatically trigger their actions - like many things in Spring, the feature has to be
declaratively enabled (which means if you ever suspect caching is to blame, you can
disable it by removing only one configuration line rather than all the annotations in
your code).

To enable caching annotations add the annotation @EnableCaching to one of your
@Configuration classes:

@Configuration@EnableCachingpublicclassAppConfig {
}

Alternatively for XML configuration use the cache:annotation-driven element:

Both the cache:annotation-driven element and @EnableCaching annotation allow various
options to be specified that influence the way the caching behavior is added to the
application through AOP. The configuration is intentionally similar with that of
@Transactional:

The default advice mode for processing caching annotations is "proxy" which allows
for interception of calls through the proxy only; local calls within the same class
cannot get intercepted that way. For a more advanced mode of interception, consider
switching to "aspectj" mode in combination with compile-time or load-time weaving.

Name of cache manager to use. A default CacheResolver will be initialized behind
the scenes with this cache manager (or `cacheManager`if not set). For more
fine-grained management of the cache resolution, consider setting the 'cache-resolver'
attribute.

cache-resolver

N/A (See CachingConfigurer javadocs)

A SimpleCacheResolver using the configured cacheManager.

The bean name of the CacheResolver that is to be used to resolve the backing caches.
This attribute is not required, and only needs to be specified as an alternative to
the 'cache-manager' attribute.

key-generator

N/A (See CachingConfigurer javadocs)

SimpleKeyGenerator

Name of the custom key generator to use.

error-handler

N/A (See CachingConfigurer javadocs)

SimpleCacheErrorHandler

Name of the custom cache error handler to use. By default, any exception throw during
a cache related operations are thrown back at the client.

mode

mode

proxy

The default mode "proxy" processes annotated beans to be proxied using Spring’s AOP
framework (following proxy semantics, as discussed above, applying to method calls
coming in through the proxy only). The alternative mode "aspectj" instead weaves the
affected classes with Spring’s AspectJ caching aspect, modifying the target class byte
code to apply to any kind of method call. AspectJ weaving requires spring-aspects.jar
in the classpath as well as load-time weaving (or compile-time weaving) enabled. (See
Spring configuration for details on how to set up
load-time weaving.)

proxy-target-class

proxyTargetClass

false

Applies to proxy mode only. Controls what type of caching proxies are created for
classes annotated with the @Cacheable or @CacheEvict annotations. If the
proxy-target-class attribute is set to true, then class-based proxies are created.
If proxy-target-class is false or if the attribute is omitted, then standard JDK
interface-based proxies are created. (See Proxying mechanisms
for a detailed examination of the different proxy types.)

order

order

Ordered.LOWEST_PRECEDENCE

Defines the order of the cache advice that is applied to beans annotated with
@Cacheable or @CacheEvict. (For more information about the rules related to
ordering of AOP advice, see Advice ordering.)
No specified ordering means that the AOP subsystem determines the order of the advice.

<cache:annotation-driven/> only looks for @Cacheable/@CachePut/@CacheEvict/@Caching
on beans in the same application context it is defined in. This means that, if you put
<cache:annotation-driven/> in a WebApplicationContext for a DispatcherServlet, it
only checks for beans in your controllers, and not your services.
See the MVC section for more information.

Method visibility and cache annotations

When using proxies, you should apply the cache annotations only to methods with
public visibility. If you do annotate protected, private or package-visible methods
with these annotations, no error is raised, but the annotated method does not exhibit
the configured caching settings. Consider the use of AspectJ (see below) if you need to
annotate non-public methods as it changes the bytecode itself.

Spring recommends that you only annotate concrete classes (and methods of concrete
classes) with the @Cache* annotation, as opposed to annotating interfaces. You
certainly can place the @Cache* annotation on an interface (or an interface method),
but this works only as you would expect it to if you are using interface-based proxies.
The fact that Java annotations are not inherited from interfaces means that if you
are using class-based proxies ( proxy-target-class="true") or the weaving-based aspect
( mode="aspectj"), then the caching settings are not recognized by the proxying and
weaving infrastructure, and the object will not be wrapped in a caching proxy, which
would be decidedly bad.

In proxy mode (which is the default), only external method calls coming in through the
proxy are intercepted. This means that self-invocation, in effect, a method within the
target object calling another method of the target object, will not lead to an actual
caching at runtime even if the invoked method is marked with @Cacheable - considering
using the aspectj mode in this case. Also, the proxy must be fully initialized to provide
the expected behaviour so you should not rely on this feature in your initialization
code, i.e. @PostConstruct.

8.3.7. Using custom annotations

Custom annotation and AspectJ

This feature only works out-of-the-box with the proxy-based approach but can be enabled
with a bit of extra effort using AspectJ.

The spring-aspects module defines an aspect for the standard annotations only. If you
have defined your own annotations, you also need to define an aspect for those. Check
AnnotationCacheAspect for an example.

The caching abstraction allows you to use your own annotations to identify what method
triggers cache population or eviction. This is quite handy as a template mechanism as it
eliminates the need to duplicate cache annotation declarations (especially useful if the
key or condition are specified) or if the foreign imports (org.springframework) are
not allowed in your code base. Similar to the rest of the
stereotype annotations, @Cacheable, @CachePut,
@CacheEvict and @CacheConfig can be used as meta-annotations,
that is annotations that can annotate other annotations. To wit, let us replace a common
@Cacheable declaration with our own, custom annotation:

Even though @SlowService is not a Spring annotation, the container automatically picks
up its declaration at runtime and understands its meaning. Note that as mentioned
above, the annotation-driven behavior needs to be enabled.

8.4. JCache (JSR-107) annotations

Since the Spring Framework 4.1, the caching abstraction fully supports the JCache
standard annotations: these are @CacheResult, @CachePut, @CacheRemove and
@CacheRemoveAll as well as the @CacheDefaults, @CacheKey and @CacheValue
companions. These annotations can be used right the way without migrating your
cache store to JSR-107: the internal implementation uses Spring’s caching abstraction
and provides default CacheResolver and KeyGenerator implementations that are
compliant with the specification. In other words, if you are already using Spring’s
caching abstraction, you can switch to these standard annotations without changing
your cache storage (or configuration, for that matter).

8.4.1. Feature summary

For those who are familiar with Spring’s caching annotations, the following table
describes the main differences between the Spring annotations and the JSR-107
counterpart:

Table 13. Spring vs. JSR-107 caching annotations

Spring

JSR-107

Remark

@Cacheable

@CacheResult

Fairly similar. @CacheResult can cache specific exceptions and force the
execution of the method regardless of the content of the cache.

@CachePut

@CachePut

While Spring updates the cache with the result of the method invocation, JCache
requires to pass it as an argument that is annotated with @CacheValue. Due
to this difference, JCache allows to update the cache before or after the
actual method invocation.

@CacheEvict

@CacheRemove

Fairly similar. @CacheRemove supports a conditional evict in case the
method invocation results in an exception.

@CacheEvict(allEntries=true)

@CacheRemoveAll

See @CacheRemove.

@CacheConfig

@CacheDefaults

Allows to configure the same concepts, in a similar fashion.

JCache has the notion of javax.cache.annotation.CacheResolver that is identical
to the Spring’s CacheResolver interface, except that JCache only supports a single
cache. By default, a simple implementation retrieves the cache to use based on
the name declared on the annotation. It should be noted that if no cache name
is specified on the annotation, a default is automatically generated, check the
javadoc of @CacheResult#cacheName() for more information.

CacheResolver instances are retrieved by a CacheResolverFactory. It is
possible to customize the factory per cache operation:

For all referenced classes, Spring tries to locate a bean with the given type. If
more than one match exists, a new instance is created and can use the regular
bean lifecycle callbacks such as dependency injection.

Keys are generated by a javax.cache.annotation.CacheKeyGenerator that serves the
same purpose as Spring’s KeyGenerator. By default, all method arguments are taken
into account unless at least one parameter is annotated with @CacheKey. This is
similar to Spring’s custom key generation
declaration. For instance these are identical operations, one using Spring’s
abstraction and the other with JCache:

The CacheKeyResolver to use can also be specified on the operation, in a similar
fashion as the CacheResolverFactory.

JCache can manage exceptions thrown by annotated methods: this can prevent an update of
the cache but it can also cache the exception as an indicator of the failure instead of
calling the method again. Let’s assume that InvalidIsbnNotFoundException is thrown if
the structure of the ISBN is invalid. This is a permanent failure, no book could ever be
retrieved with such parameter. The following caches the exception so that further calls
with the same, invalid ISBN, throws the cached exception directly instead of invoking
the method again.

8.4.2. Enabling JSR-107 support

Nothing specific needs to be done to enable the JSR-107 support alongside Spring’s
declarative annotation support. Both @EnableCaching and the
cache:annotation-driven element will enable automatically the JCache support
if both the JSR-107 API and the spring-context-support module are present in
the classpath.

Depending of your use case, the choice is basically yours. You can even mix
and match services using the JSR-107 API and others using Spring’s own
annotations. Be aware however that if these services are impacting the same
caches, a consistent and identical key generation implementation should be used.

8.5. Declarative XML-based caching

If annotations are not an option (no access to the sources or no external code), one can
use XML for declarative caching. So instead of annotating the methods for caching, one
specifies the target method and the caching directives externally (similar to the
declarative transaction management advice).
The previous example can be translated into:

In the configuration above, the bookService is made cacheable. The caching semantics
to apply are encapsulated in the cache:advice definition which instructs method
findBooks to be used for putting data into the cache while method loadBooks for
evicting data. Both definitions are working against the books cache.

The aop:config definition applies the cache advice to the appropriate points in the
program by using the AspectJ pointcut expression (more information is available in
Aspect Oriented Programming with Spring).
In the example above, all methods from the BookService are considered and
the cache advice applied to them.

The declarative XML caching supports all of the annotation-based model so moving between
the two should be fairly easy - further more both can be used inside the same
application. The XML based approach does not touch the target code however it is
inherently more verbose; when dealing with classes with overloaded methods that are
targeted for caching, identifying the proper methods does take an extra effort since the
method argument is not a good discriminator - in these cases, the AspectJ pointcut can
be used to cherry pick the target methods and apply the appropriate caching
functionality. However through XML, it is easier to apply a package/group/interface-wide
caching (again due to the AspectJ pointcut) and to create template-like definitions (as
we did in the example above by defining the target cache through the cache:definitionscache attribute).

8.6. Configuring the cache storage

Out of the box, the cache abstraction provides several storage integration. To use
them, one needs to simply declare an appropriate CacheManager - an entity that
controls and manages Caches and can be used to retrieve these for storage.

8.6.1. JDK ConcurrentMap-based Cache

The JDK-based Cache implementation resides under
org.springframework.cache.concurrent package. It allows one to use ConcurrentHashMap
as a backing Cache store.

The snippet above uses the SimpleCacheManager to create a CacheManager for the two
nested ConcurrentMapCache instances named default and books. Note that the
names are configured directly for each cache.

As the cache is created by the application, it is bound to its lifecycle, making it
suitable for basic use cases, tests or simple applications. The cache scales well and is
very fast but it does not provide any management or persistence capabilities nor
eviction contracts.

8.6.2. Ehcache-based Cache

Ehcache 3.x is fully JSR-107 compliant and no dedicated support is required for it.

The Ehcache 2.x implementation is located under org.springframework.cache.ehcache package.
Again, to use it, one simply needs to declare the appropriate CacheManager:

This setup bootstraps the ehcache library inside Spring IoC (through the ehcache bean) which
is then wired into the dedicated CacheManager implementation. Note the entire
ehcache-specific configuration is read from ehcache.xml.

8.6.3. Caffeine Cache

Caffeine is a Java 8 rewrite of Guava’s cache and its implementation is located under
org.springframework.cache.caffeine package and provides access to several features
of Caffeine.

Configuring a CacheManager that creates the cache on demand is straightforward:

8.6.6. Dealing with caches without a backing store

Sometimes when switching environments or doing testing, one might have cache
declarations without an actual backing cache configured. As this is an invalid
configuration, at runtime an exception will be thrown since the caching infrastructure
is unable to find a suitable store. In situations like this, rather then removing the
cache declarations (which can prove tedious), one can wire in a simple, dummy cache that
performs no caching - that is, forces the cached methods to be executed every time:

The CompositeCacheManager above chains multiple CacheManagers and additionally,
through the fallbackToNoOpCache flag, adds a no op cache that for all the
definitions not handled by the configured cache managers. That is, every cache
definition not found in either jdkCache or gemfireCache (configured above) will be
handled by the no op cache, which will not store any information causing the target
method to be executed every time.

8.7. Plugging-in different back-end caches

Clearly there are plenty of caching products out there that can be used as a backing
store. To plug them in, one needs to provide a CacheManager and Cache implementation
since unfortunately there is no available standard that we can use instead. This may
sound harder than it is since in practice, the classes tend to be simple
adapters that map the caching abstraction
framework on top of the storage API as the ehcache classes can show. Most
CacheManager classes can use the classes in org.springframework.cache.support
package, such as AbstractCacheManager which takes care of the boiler-plate code
leaving only the actual mapping to be completed. We hope that in time, the libraries
that provide integration with Spring can fill in this small configuration gap.

8.8. How can I set the TTL/TTI/Eviction policy/XXX feature?

Directly through your cache provider. The cache abstraction is…​ well, an abstraction
not a cache implementation. The solution you are using might support various data
policies and different topologies which other solutions do not (take for example the JDK
ConcurrentHashMap) - exposing that in the cache abstraction would be useless simply
because there would no backing support. Such functionality should be controlled directly
through the backing cache, when configuring it or through its native API.

9. Appendix

9.1. XML Schemas

This part of the appendix lists XML schemas related to integration technologies.

9.1.1. The jee schema

The jee tags deal with Java EE (Java Enterprise Edition)-related configuration issues,
such as looking up a JNDI object and defining EJB references.

To use the tags in the jee schema, you need to have the following preamble at the top
of your Spring XML configuration file; the text in the following snippet references the
correct schema so that the tags in the jee namespace are available to you.

In the interest of completeness, to use the tags in the jms schema, you need to have
the following preamble at the top of your Spring XML configuration file; the text in the
following snippet references the correct schema so that the tags in the jms namespace
are available to you.

To use the tags in the cache schema, you need to have the following preamble at the
top of your Spring XML configuration file; the text in the following snippet references
the correct schema so that the tags in the cache namespace are available to you.